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+---
+title: "Setting up an email server in 2020 with OpenSMTPD and Dovecot: extras"
+date: 2020-04-28
+layout: "blog"
+toc: true
+---
+
+This sequel to my post
+"[Setting up an email server in 2020 with OpenSMTPD and Dovecot](/blog/2020/email-server/)"
+gives extra tips and tricks to extend your email setup.
+See also the sequel's sequel,
+"[Revisiting my email server in 2022](/blog/2022/email-server-revisited/)".
+
+Last updated on 2022-09-12.
+
+
+## General
+
+### Multiple domains
+
+You can generalize your setup to handle multiple domains
+with very little effort. In the following,
+I'll assume that your two domains are called `foo.com` and `bar.com`.
+
+
+#### DNS records
+
+There should be MX, SPF, DKIM and DMARC records for both domains,
+as explained in the previous guide. Fortunately, these records
+can have identical contents for both domains!
+
+However, it remains essential that the mail server's mailname
+and reverse DNS domain name match up exactly,
+so you should create MX records with that in mind.
+Therefore, if the email server for both domains has `mx1.foo.com`
+as reverse DNS name, the MX records should look like this:
+```sh
+foo.com. MX 42 mx1.foo.com.
+bar.com. MX 42 mx1.foo.com.
+```
+This is perfectly valid: the only thing that matters is that
+what your SMTP server calls itself agrees with what reverse DNS
+says that the server is actually called.
+
+
+#### Dovecot
+
+To make Dovecot aware of multiple domains,
+you only need to update the `/etc/dovecot/users` file
+to add accounts for both domains.
+However, in the original guide, I said to only write `user`
+in the file, without the `@foo.com`, for an address `user@foo.com`.
+Unsurprisingly, that isn't an option for multiple domains,
+so you must put the full address in `/etc/dovecot/users`.
+
+Then update `/etc/dovecot/dovecot.conf` to reflect that,
+by changing `%n` to `%u` in `username_format`:
+```sh
+userdb {
+	driver = passwd-file
+	args = username_format=%u /etc/dovecot/users
+	override_fields = uid=vmail gid=vmail home=/home/vmail/%d/%n
+}
+```
+Also note the change in the `home` setting:
+the inbox of a user `user@foo.com` will now be stored
+in `/home/vmail/foo.com/user`.
+That's all you need to change.
+
+
+#### OpenSMTPD
+
+To inform OpenSMTPD of all the domains,
+create a new file `/etc/smtpd/domains`,
+and in there put all desired names on their own line:
+```sh
+foo.com
+bar.com
+```
+And as I mentioned when discussing the DNS records,
+you should check that `/etc/smtpd/mailname` agrees
+with your server's reverse DNS.
+
+Then, in the main configuration file, tell OpenSMTPD to
+use the new domains file when deciding whether to accept an message,
+by declaring a new table and changing the `match` line for inbound mail:
+```sh
+table domains "/etc/smtpd/domains"
+# ...
+match from any for domain <domains> action "RECV"
+```
+
+#### Rspamd
+
+The last thing to do is to inform Rspamd of the multiple domains.
+It's really easy: simply add multiple domain blocks:
+```c
+domain {
+	foo.com {
+		path = "/path/to/dkim/private.key";
+		selector = "hello";
+	}
+}
+domain {
+	bar.com {
+		path = "/path/to/dkim/private.key";
+		selector = "world";
+	}
+}
+```
+
+### Advanced security
+
+SPF, DKIM and DMARC are email's traditional DNS-based security systems,
+but in 2018 the IETF released [RFC 8460](https://tools.ietf.org/html/rfc8460) and [RFC 8461](https://www.rfc-editor.org/rfc/rfc8461.txt),
+which respectively define TLSRPT and MTA-STS,
+two fancy new systems focused on TLS-encrypted email transport.
+
+These security mechanisms are pretty new,
+so you won't get a huge benefit from enabling them,
+but big email providers' draconian spam filters might like it.
+
+
+#### TLSRPT
+
+TLS reporting, or TLSRPT for short, is very simple:
+all it does is provide a contact email address in case
+somebody has trouble with the TLS configuration of your SMTP server.
+
+To enable it for your custom email domain `example.com`,
+simply create a DNS TXT record for the `_smtp._tls` subdomain:
+```sh
+_smtp._tls.example.com. TXT "v=TLSRPTv1; rua=mailto:<contact>"
+```
+Where `<contact>` is an email address of your choosing.
+That's all!
+
+
+#### MTA-STS
+
+MTA Strict Transport Security (MTA-STS) tells other servers
+that you take TLS encryption of messages very seriously,
+so they should avoid sending you unencrypted email,
+and should only accept certain certificates from your side.
+
+Compared to the previously discussed DNS-based security extensions,
+MTA-STS is a bit more work to set up,
+because you'll also need an HTTP web server.
+
+The DNS part is still pretty simple:
+create yet another DNS TXT record,
+this time for the subdomain `_mta-sts`:
+```sh
+_mta-sts.example.com. TXT "v=STSv1; id=<id>"
+```
+The `<id>` should identify the version of your policy,
+so other servers can quickly see if something changed.
+I recommend using today's date.
+
+For the next part, I'll assume that you already have
+a web server running on a server with the IP address `1.2.3.4`.
+I use [nginx](https://nginx.org/) for this, running
+on the same server as OpenSMTPD and Dovecot,
+but you don't have to do the same.
+
+Create an A record which binds your server
+to the subdomain `mta-sts` (without underscore):
+```sh
+mta-sts.example.com. A 1.2.3.4
+```
+Set your web server to serve the file
+`https://mta-sts.example.com/.well-known/mta-sts.txt`
+(we'll discuss that file in a moment).
+Note that this policy file **must** be served over HTTPS,
+so you need a valid TLS certificate for that domain.
+
+The contents of the `mta-sts.txt` policy file are as follows,
+where `mx1.example.com` and `mx2.example.com` are the hosts
+mentioned in `example.com`'s DNS MX records:
+```sh
+version: STSv1
+mode: enforce
+mx: mx1.example.com
+mx: mx2.example.com
+max_age: <age>
+```
+All MX servers must be mentioned this way.
+If you're feeling cautious, you may want to set
+`mode` to `testing` in the beginning.
+This policy is valid for `<age>` seconds,
+which is recommended to be several weeks,
+but to start with, I suggest using 86400 seconds (one day).
+Finally, ensure that this file has CRLF Windows-style line endings.
+
+To correctly pass an MTA-STS test, the TLS certificate
+presented by e.g. `mx1.example.com` should be valid for `mx1.exaple.com`.
+To achieve this without needing to manage too many certificates,
+you can specify multiple domains when requesting a certificate,
+or you can use a wildcard domain (`*.example.com`).
+Note, however, that MTA-STS testing tools don't like
+the latter option, so I recommend the former.
+
+Once you're done, check your work by using either
+[ESMTP](https://esmtp.email/tools/mta-sts/)'s or [Ayke](https://aykevl.nl/apps/mta-sts/)'s
+online MTA-STS validation tools,
+ignoring any warnings about DNSSEC or DANE.
+If all is good, great!
+
+Even if you did everything correctly,
+these tools will warn you that you're not using DNSSEC/DANE.
+It might then be tempting to set that up for even more security,
+but I recommend against that for private servers: take a look at [this](https://dane.sys4.de/common_mistakes).
+
+
+
+## OpenSMTPD
+
+### Client certificates (in addition to passwords)
+
+You can configure OpenSMTPD to request a client certificate
+for sending emails, as a second factor for authentication.
+
+UPDATE: When I wrote this two years ago, it worked,
+but now it doesn't anymore, and I can't figure out why.
+It seems OpenSMTPD always rejects the client certificates for being self-signed,
+even if they can manually be verified for our CA using the `openssl` tool.
+I'm leaving this tutorial here for anyone who's interested,
+but it's unlikely I'll fix it anytime soon.
+
+
+#### Certificates
+
+We need to start with some cryptography to create and verify certificates.
+I recommend that you do all of this on your trusted  *client* device,
+and only copy the necessary files to the server later.
+
+DISCLAIMER:
+All the keys and certificates that we'll generate in this section are
+for **private use** only, to handle a small number of trusted clients.
+I'm not a cryptography expert, so you should **not** listen to me
+for large-scale systems that may involve untrusted devices.
+
+The first step is to set up a private Certificate Authority (CA),
+which issues the client certificates and can be used to verify them.
+Start by generating an RSA private key,
+which you should store in a safe place and not share with anyone:
+```sh
+$ openssl genrsa -out mailca.key 2048
+```
+Extract a public certificate from this key as follows.
+Because we're lazy, we give it a lifetime of 36500 days:
+```sh
+$ openssl req -new -x509 -days 36500 -key mailca.key -out mailca.crt
+```
+When running this command, OpenSSL will ask you some questions
+about who this certificate is intended for.
+Since this is for personal use, your answers don't matter,
+so just use the defaults.
+Some fields (I think only *Country Name* and *Organization Name*)
+cannot be empty, but the others can.
+
+Moving on to the client, once again generate an RSA private key:
+```sh
+$ openssl genrsa -out mailclient.key 2048
+```
+From this private key, create a Certificate Signing Request (CSR)
+as follows, where you'll be asked the same questions as before:
+```sh
+$ openssl req -new -key mailclient.key -out mailclient.csr
+```
+By feeding this CSR to the CA, we can create a signed client certificate
+that can be verified using the CA's public certificate.
+```sh
+$ openssl x509 -req -in mailclient.csr -out mailclient.crt \
+        -days 36499 -CA mailca.crt -CAkey mailca.key
+```
+If you want to multiple client certificates,
+just repeat the last few steps for each one.
+
+
+#### Server
+
+OpenSMTPD needs to verify the validity of client certificates
+using the CA's public certificate, so you should copy that
+to somewhere on the server, e.g. `/etc/smtpd/mailca.crt`,
+and declare it to OpenSMTPD by adding this near
+the top of `/etc/smtpd/smtpd.conf`:
+```sh
+ca "mailca" cert "/etc/smtpd/mailca.crt"
+```
+Then replace the entire configuration for outbound mail as follows.
+Note that this removes SMTPS support, leaving only STARTTLS:
+```sh
+# Outbound
+listen on eth0 port 587 tls-require verify pki "example.com" ca "mailca" auth <passwds> filter "rspamd"
+action "SEND" relay srs
+match from any auth for any action "SEND"
+```
+The magic word here is "`verify`", which tells OpenSMTPD
+to ask for a client certificate and to verify it using the given CA.
+
+
+#### Client
+
+Now you won't be able to send emails if your client doesn't
+present its certificate to the server!
+Unfortunately, not all mail clients support this; personally
+I use [Thunderbird](https://www.thunderbird.net/) with success.
+I won't include any client-specific configuration here,
+but I will say this:
+
+For some clients (like Thunderbird), you'll have an easier time
+importing your client certificate if you encode it in the
+[PKCS #12](https://en.wikipedia.org/wiki/PKCS_12) storage format:
+```sh
+$ openssl pkcs12 -export -in mailclient.crt -inkey mailclient.key \
+        -certfile mailca.crt -out mailclient.pfx
+```
+OpenSSL will ask you to set a password, which you'll need to
+enter again when importing the certificate into the client.
+
+
+
+### ~~Client certificates (instead of passwords)~~
+
+UPDATE: Don't do this.
+As said above, OpenSMTPD's certificate verification is a mystery,
+so for all I know, if you follow the instructions in this subsection,
+you might find yourself running an *open* SMTP relay!
+That would be bad, because anyone on the Internet
+could send emails through your server with zero authentication.
+In theory, the client certificates act as authentication,
+but, again, the verification process is mysterious,
+so I'm just not confident enough to say.
+
+If you really want to, you can use the client certificates
+as a substitute for passwords. This is especially useful
+if you set up a catchall inbox in Dovecot,
+because this will allow you to send emails
+from arbitrary addresses from your domain.
+
+To do this, follow the same procedure as in the previous section,
+but with a slightly different OpenSMTPD configuration:
+```sh
+listen on eth0 port 587 tls-require verify pki "example.com" ca "mailca" filter "rspamd" tag "VALID"
+action "SEND" relay srs
+match from any tag "VALID" for any action "SEND"
+```
+All incoming connections that present a good certificate
+will be tagged as being `VALID`, and their mail will be relayed.
+
+Unfortunately, we're not quite done yet here,
+because Rspamd is now very confused...
+
+
+#### Rspamd
+
+When OpenSMTPD passes a message through Rspamd, it also includes
+some metadata, most notably whether the sender has authenticated
+successfully with OpenSMTPD... which is now no longer the case
+for submissions, because we've removed the `auth` directive!
+
+Rspamd therefore starts regarding these outgoing emails
+as *incoming* emails, because they don't seem
+to come from a trusted user. So instead of signing them with DKIM
+and handing them back to OpenSMTPD, it will do a full spam scan.
+If they get a high spam score (which is likely for short test emails),
+*your* spam filter, running on *your* server,
+will be flagging *your* messages as spam!
+
+The solution is to whitelist your domain(s) in Rspamd,
+so it won't scan them. To do this, create a new file
+`/etc/rspamd/local.d/settings.conf` with these contents,
+where `foo.com` and `bar.com` are the domains to whitelist:
+```c
+outbound {
+	priority = high;
+	from = "@foo.com";
+	from = "@bar.com";
+	apply {
+		actions {
+			add_header = 1000;
+		}
+	}
+}
+```
+Setting `priority` to `high` ensures that Rspamd checks
+this rule before doing anything else.
+You can add any number of `from` directives;
+this rule will be applied if any of them match.
+It only sets the threshold for the action `add_header` to `1000`.
+That is, if the email doesn't get a spam score of at least 1000
+(the default is 6) Rspamd will not add any spam tags.
+
+Because Rspamd is still regarding your emails as inbound,
+you also need to change the global settings of
+the DKIM signer in `/etc/rspamd/local.d/dkim_signing.conf`,
+such that they include the following:
+```c
+sign_inbound = true;
+allow_hdrfrom_mismatch  = true;
+allow_username_mismatch = true;
+```
+This tells Rspamd to add DKIM signatures to incoming emails,
+which in this case includes yours.
+Allowing these mismatches ensures that the messages still get signed,
+even if you're sending from an arbitrary address.
+
+
+
+## Dovecot
+
+### Catchall inbox
+
+In Dovecot, you can create a catch-all inbox that will accept all
+emails sent to your domain that don't match anyone in `/etc/dovecot/users`.
+Just add another `userdb` block *after* the first:
+```sh
+userdb {
+	driver = static
+	args = uid=vmail gid=vmail home=/var/vmail/catchall allow_all_users=yes
+}
+```
+The `static` driver means there is no table file:
+all configuration is directly within this `userdb` block.
+If we don't specify `allow_all_users=yes`, then Dovecot
+will check whether users exist using the `passdb` table,
+and will conclude that the recipient is invalid.
+
+
+
diff --git a/source/blog/2020/email-server/index.md b/source/blog/2020/email-server/index.md
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index 0000000..962e6aa
--- /dev/null
+++ b/source/blog/2020/email-server/index.md
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+---
+title: "Setting up an email server in 2020 with OpenSMTPD and Dovecot"
+date: 2020-04-27
+layout: "blog"
+toc: true
+---
+
+So, you want to set up your own email server? In that case, welcome.
+
+There are many reasons to run a custom email server,
+ranging from privacy concerns about providers like Google,
+to just wanting to do it for fun and/or learning.
+Since you're here, I assume you've already found a reason.
+
+Beware: this is a messy topic, and the available documentation
+is even messier, so it could take a while before you get it to work properly.
+I've compiled this guide according to my experiences
+in an attempt to make this dark art more accessible,
+but your mileage may vary considerably. I hope you find it useful.
+
+This guide is aimed at people who are comfortable with
+the Linux/*BSD command line.
+
+When you're done (if you get that far), take a look at the sequels
+"[Setting up an email server in 2020 with OpenSMTPD and Dovecot: extras](/blog/2020/email-server-extras/)"
+and "[Revisiting my email server in 2022](/blog/2022/email-server-revisited/)"
+for ideas on how to extend your setup.
+
+Last updated on 2022-09-12.
+
+
+
+## Preparation
+
+Setting up email is relatively complex compared to e.g. a static website,
+because you need to configure not one, but *two* server programs,
+*and* you need to shoehorn modern security features into email's Stone-Age design.
+I'll start by explaining the general structure of a mail server setup.
+
+
+
+### How email works
+
+The programs involved in the exchange of emails are called [agents](https://en.wikipedia.org/wiki/Email_agent_(infrastructure)).
+Officially, there are 5 different types of agent: MUA, MSA, MTA, MDA and MRA.
+But fortunately, it's reasonable to treat the MRA and MSA
+as being part of the MUA and MTA, respectively.
+
+The *Mail User Agent* (MUA) is simply the client on your device at home
+that you use to send and receive emails, and this guide assumes
+you already have a favourite program for this, e.g. [Thunderbird](https://www.thunderbird.net/en-US/).
+Nowadays it's fashionable to use a web interface for emails,
+but that's also beyond the scope of this guide.
+
+The *Mail Delivery Agent* (MDA) is a program that watches over
+the server's copy of your mailbox: it manages your inbox,
+remembers which messages you have or haven't read,
+keeps a copy of your drafts, etc.
+When you open your mailbox, your MUA will connect to
+your server's MDA using the [IMAP](https://en.wikipedia.org/wiki/Internet_Message_Access_Protocol) protocol
+(or [POP3](https://en.wikipedia.org/wiki/Post_Office_Protocol), but that one's [obsolete](https://pop2imap.com/)).
+
+The *Mail Transfer Agent* (MTA) is responsible for
+making messages arrive at the right destination.
+When you send an email, your MUA will pass it on to your server's MTA,
+which will in turn pass it on to the recipient's mail server.
+Likewise, when someone sends *you* an email from another server,
+the MTA will receive it and hand it over to the MDA so you can read it later.
+In both cases the MTA speaks the [SMTP](https://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol) protocol.
+
+In this guide our MDA will be [Dovecot](https://dovecot.org/),
+which is a very popular choice for that role.
+As for the MTA, there exist several options,
+the most popular being [Postfix](http://www.postfix.org/) and [Exim](https://exim.org/).
+However, this guide uses the newer, lesser-known [OpenSMTPD](https://opensmtpd.org/),
+which in my experience is *much* easier to set up:
+Postfix and Exim have complex configurations and
+are geared towards large-scale email providers,
+whereas OpenSMTPD is more beginner-friendly.
+
+
+
+### Security
+
+The base email system is horribly insecure on its own,
+so we still need to duct-tape on some security features.
+In this context, "security" has two meanings:
+spam protecion and privacy protection (encryption).
+
+Spam protection also means two things here:
+defending yourself against spammers, and
+preventing that *your* emails get flagged as spam.
+The former is optional, but the latter is not:
+big providers such as Google and Microsoft
+use infamously strict spam filters,
+and if they decide that your server is a spammer,
+there's almost nothing you can do about it.
+Spam protection techniques will be discussed
+in more detail over the course of this guide.
+
+Privacy protection is important in the 21st century:
+you don't want a random router in the Internet to read all your emails,
+which may contain sensitive information such as
+private conversations and account password reset links.
+You should therefore try to make sure that emails are
+transported over an encrypted channel.
+To do this, you have two options for encryption:
+*mandatory* and *opportunistic* encryption.
+
+Mandatory encryption is only practical for client-server
+communication (not server-server), and is provided by IMAPS and SMTPS,
+which wrap the IMAP and SMTP protocols in [TLS](https://en.wikipedia.org/wiki/Transport_Layer_Security),
+in the same way that [HTTPS](https://en.wikipedia.org/wiki/HTTPS) does for [HTTP](https://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol).
+
+For server-server communication, the only option is
+opportunistic encryption in the form of [STARTTLS](https://en.wikipedia.org/wiki/STARTTLS),
+where communication is only encrypted if both parties agree
+after a short unencrypted discussion.
+That last part is vulnerable to [MitM](https://en.wikipedia.org/wiki/Man-in-the-middle_attack) attacks,
+where anyone along the path of the email servers' discussion
+can alter the exchange to block the use of encryption,
+which sometimes actually [happens](https://www.eff.org/deeplinks/2014/11/starttls-downgrade-attacks) in practice.
+
+The only way to make sure that STARTTLS is used in that case
+is to refuse any exchange unless the servers agree to use encryption.
+Unfortunately, that's a risky approach that I can't recommend,
+because not all servers support encryption (unbelievable, right?).
+For example, I've received airline booking confirmations,
+full of personal details, and made with billion-dollar companies,
+sent across the Internet without any protection.
+
+This guide includes intructions to enable encryption,
+but assumes that you already have a TLS certificate for that.
+If not, find a guide to get one from [Let's Encrypt](https://letsencrypt.org/) (it's free!),
+and remember that you'll need to renew it every few months.
+Using a self-signed certificate *may* work, but I don't recommend it.
+
+In the rest of this guide I'll assume that
+you have a public full-chain TLS certificate at `/etc/ssl/certs/example.com.pem`,
+and a private encryption key at `/etc/ssl/private/example.com.pem`.
+
+
+
+### Server
+
+Obviously, you'll need a server to run the MTA and MDA on.
+You can host your own at home, but the more reliable option is
+to rent one in a data center ([VPS](https://en.wikipedia.org/wiki/Virtual_private_server)).
+This guide was written with a Linux server in mind,
+but in theory it should also work on the BSDs
+([OpenBSD](https://www.openbsd.org/), [FreeBSD](https://www.freebsd.org/),
+[NetBSD](https://www.netbsd.org/), etc.) with minimal adaptation.
+
+The server must be online 24/7, you must have root SSH access,
+it must have a static IP address, and TCP network port 25 must be open.
+Especially check that last one: you may need to explicitly ask
+your home ISP or the server provider to enable port 25,
+because they often close it to prevent spam.
+You can usually do this from their web interface.
+
+You also must have a domain name, which I'll call `example.com`.
+This will be necessary for basically everything:
+DNS records, TLS certificate, MTA network configuration, etc.
+If you don't have one yet, you can choose between many registrars
+to rent one from. Personally I use and can recommend [Gandi](https://www.gandi.net/).
+
+Note that it's a **bad** idea to use a domain like `foo.bar.com`,
+where you control the `foo` part but *not* the `bar` part:
+in that case, a spammer in control of `qux.bar.com`
+could negatively affect *your* reputation
+in the eyes of other email providers.
+
+Lastly, when setting up an email server, you also have the choice
+between using to *system* users or *virtual* users.
+With system users, if an email arrives for `john@example.com`,
+then the MTA and MDA will expect that there exists
+a `john` Unix user on the server to deliver it to.
+With virtual users, you have much more flexibility, so that's what we'll use.
+All email will be managed under a single Unix user/group called `vmail`.
+Create it as follows:
+```sh
+# GNU CoreUtils:
+$ groupadd vmail
+$ useradd -g vmail vmail
+# BusyBox:
+$ addgroup vmail
+$ adduser -D -G vmail vmail
+# *BSD:
+$ no clue, but it should be similar
+```
+
+
+## DNS records
+
+Now we must set up all the necessary DNS records, which
+is usually possible from the domain registrar's web interface.
+It may take a while for your changes to propagate over the Internet,
+so I recommend doing this section now and the rest tomorrow.
+
+Firstly, you should already have an A and/or AAAA record
+to associate your domain `example.com` with the server's IP address.
+For email it is **essential** that you also have [reverse DNS](https://en.wikipedia.org/wiki/Reverse_DNS_lookup)
+set up correctly. If you're renting your server remotely,
+you can often do this from the provider's configuration tool,
+otherwise, you should create a PTR-type DNS record,
+although that's beyond the scope of this guide.
+
+Once you're done, I recommend testing your DNS records
+using the [MX Lookup](https://mxtoolbox.com/MXLookup.aspx) online tool.
+
+
+
+### MX
+
+To inform the rest of the Internet that your server is an email server,
+create an MX (Mail eXchanger) DNS record for your domain.
+Note the dot at the end of the domain name:
+```sh
+example.com. MX 42 example.com.
+```
+When a message is sent to an email address ending in `@example.com`,
+the sender will query DNS for any MX records for `example.com`.
+There it will find a domain name (in this case `example.com` again),
+for which it will look up the IP address using an A/AAAA record.
+The domain name in the record must **not** have an associated CNAME record;
+it must be directly translatable to an IP address.
+
+You may have multiple MX records, containing different domain names,
+each with a preference number (`42` in the example above).
+The sender will try MX records with *lower* numbers first,
+and if that server is unavailable, it will try a higher number.
+If you have multiple mail servers (which is a good idea),
+you can thus declare those as follows:
+```sh
+example.com. MX 13 mx1.example.com.
+example.com. MX 42 mx2.example.com.
+```
+Here, a server sending an email to your domain `example.com`
+will try to send it to the IP address of `mx1.example.com` first,
+and if that fails, it will move on to `mx2.example.com`.
+If both `mx1` and `mx2` have the same number, then the sender
+will randomly choose one, which is useful for load balancing,
+although that's probably overkill for a private server.
+
+
+
+### SPF
+
+The [Sender Policy Framework](https://en.wikipedia.org/wiki/Sender_Policy_Framework) (SPF),
+is a feature which helps prevent spammers from impersonating
+your server in an attempt to get around blacklists.
+This security feature is **required** nowadays:
+if you don't use it, you'll probably get flagged as spam.
+
+SPF works by specifying which IP addresses are authorized
+to send emails from your domain name.
+You must publish this information in a
+TXT-type DNS record (**not** SPF-type, which also exists!) with the following contents:
+```sh
+example.com. TXT "v=spf1 mx -all"
+```
+Everything after the version `v=spf1` is a list of *verification mechanisms*
+for a spam filter to try out in the given order.
+The `-all` at the end says to reject your email
+if all of the previous mechanisms fail.
+See the [SPF spec](https://tools.ietf.org/html/rfc7208) for details.
+
+I recommend only using the `mx` mechanism, which tells the verifier
+to look at the A/AAAA addresses of the domains in your MX records.
+This allows you to add, remove, or change your servers
+without needing to update this record.
+
+
+
+### DKIM
+
+Then we have [DomainKeys Identified Mail](https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail) (DKIM),
+which is a more comprehensive form of anti-impersonation,
+and, like SPF, is practically **mandatory** in the modern era.
+
+It adds a cryptographic signature to all emails from your server,
+which the receiver's spam filter will verify using the email's contents,
+and a public key that you need to publish in a DNS record.
+Again, you should implement *both* SPF and DKIM, despite their overlap.
+
+To set up DKIM, create an RSA keypair, using the `openssl` utility:
+```sh
+$ openssl genrsa -out /path/to/dkim/private.key 2048
+$ openssl rsa -in /path/to/dkim/private.key -out /path/to/dkim/public.key
+```
+The minimum size is 1024 bits, but I recommend 2048 bits.
+Bigger is better, but because DNS is involved you can't stretch it
+to 4096 bits without causing discomfort to [some](https://serverfault.com/questions/747185/dkim-can-i-use-a-rsa-key-larger-than-2048bit-i-e-4096) servers.
+And I think it goes without saying that you should keep the private key private.
+
+Importantly, the DKIM DNS record **cannot** be
+attached directly to your domain `example.com`;
+instead, it should belong to a subdomain of the form
+`<selector>._domainkey.example.com`,
+where `<selector>` is an alphanumeric string you can choose (e.g. today's date),
+just remember your choice for later when configuring the DKIM signer.
+And if you change your key, keep the old record around
+for a while so old emails can still be verified.
+
+Your DKIM policy must be published in a TXT record
+as follows, where `<pubkey>` is the public RSA key `MI...AB`
+stored in `/path/to/dkim/public.key`, with the newlines removed:
+```sh
+<selector>._domainkey.example.com. TXT "v=DKIM1; t=s; h=sha256; p=<pubkey>"
+```
+Here, `v=DKIM1` is the version and must be the first tag.
+The flag `t=s` enables strict mode, as recommended by the [DKIM spec](https://tools.ietf.org/html/rfc6376),
+meaning that emails sent from subdomains immediately fail verification.
+The optional tag `h=sha256` blocks the use of the old SHA1 algorithm.
+
+
+
+### DMARC
+
+Lastly, we have [Domain-based Message Authentication, Reporting and Conformance](https://en.wikipedia.org/wiki/DMARC) (DMARC),
+which is *technically* optional, but *highly* recommended,
+because it will make you look more legitimate in the eyes of Google and Microsoft.
+It can modify the behaviour of SPF and DKIM,
+and also provides advice about what a receiver should do
+if one of your emails fails verification.
+
+To enable it, create yet another TXT record, which,
+similarly to DKIM, **must** belong to the subdomain `_dmarc.example.com`,
+and give it the following contents,
+where `<admin>` is an email address of your choosing,
+which may or may not belong to your domain:
+```sh
+_dmarc.example.com. TXT "v=DMARC1; p=reject; sp=reject; pct=100; aspf=s; adkim=s; fo=1; ruf=mailto:<admin>"
+```
+The version tag `v=DMARC1` must come first,
+followed by `p=` and `sp=`, which control what to do to unverified messages
+coming from the main domain and subdomains, respectively.
+Here, `reject` means that delivery should be refused,
+`none` asks to let it through anyway, and `quarantine` tells
+the filter to take a closer look or to put it in a spam folder.
+The percentage `pct=100` says how many of your emails to apply the policy to.
+Next, `aspf=s` and `adkim=s` enable strict mode for SPF and DKIM,
+which blocks subdomains from passing.
+Finally, `fo=1` asks for a forensic report if verification fails,
+and `ruf=` gives an address to send it to.
+If in doubt, see the [DMARC spec](https://tools.ietf.org/html/rfc7489).
+
+
+
+## MDA: Dovecot
+
+[Dovecot](https://www.dovecot.org/) is a very popular IMAP server,
+focused on being lightweight, simple, and secure,
+and has extensive and up-to-date documentation.
+It's very flexible and scalable, and keeps up well
+with the lastest security best-practices.
+
+If you installed Dovecot via a package manager,
+you'll probably have lots of configuration files
+in the `/etc/dovecot` directory.
+I want you to delete all of them. Yes, `rm -rf` that crap.
+Dovecot is simple to configure, and doesn't care where
+you put its settings, so having all that chaos in `/etc/dovecot`
+just makes things unnecessarily confusing.
+
+
+
+### Network
+
+Create a new configuration file `/etc/dovecot/dovecot.conf`,
+and start by filling in the details of your TLS certificate,
+making clear that unencrypted connections are unacceptable:
+```sh
+ssl = required
+ssl_cert = </etc/ssl/certs/example.com.pem
+ssl_key  = </etc/ssl/private/example.com.key
+
+ssl_min_protocol = TLSv1.2
+ssl_prefer_server_ciphers = yes
+
+disable_plaintext_auth = yes
+```
+The final `disable_plaintext_auth` option tells Dovecot
+to reject any passwords that were sent unencrypted.
+This means it must be [hashed](https://en.wikipedia.org/wiki/Cryptographic_hash_function)
+or sent over an encrypted connection, or both.
+
+Next, tell Dovecot which protocols to use
+and where to expect them as follows:
+```sh
+protocols = lmtp imap
+
+service lmtp {
+	unix_listener lmtp {
+		user  = vmail
+		group = vmail
+	}
+}
+
+service imap-login {
+	inet_listener imap {
+		port = 143
+	}
+	inet_listener imaps {
+		port = 993
+	}
+}
+```
+LMTP is the Local Mail Transport Protocol, which is basically SMTP
+but for exchanges within a single server or over a trusted network.
+When an email is received, Dovecot will start a child process
+under the `vmail` user/group to deliver the message to its recipient.
+
+Since we set `ssl = required` earlier, clients will only get their mail
+if the STARTTLS handshake was successful during the IMAP exchange,
+or if they connect via IMAPS to force the use of encryption.
+You can therefore optionally remove one of the two
+`inet_listener`s according to your preferences.
+
+
+
+### Users
+
+Next, we need to inform Dovecot which email addresses it should handle,
+and what to do with their messages. Create a file `/etc/dovecot/users` for this,
+which describes a user on each line in a similar format as `/etc/passwd`:
+```sh
+user:password:uid:gid::homedir
+```
+In this guide, we're using the `vmail` user for all accounts,
+so leave the `uid`, `gid`, and `homedir` fields blank.
+We'll be storing all emails in `vmail`'s home directory.
+The `user` field should be the email address excluding the `@example.com`
+(in fact, you *can* include it, but this guides assumes a small-scale
+server managing only one domain, so we exclude it).
+Create the password hash to put in the `password` field as follows:
+```sh
+# If your server is fast and has lots of RAM:
+$ doveadm pw -s ARGON2ID-CRYPT
+# If you're using a potato:
+$ doveadm pw -s SHA512-CRYPT
+```
+After you've entered your password, simply copy-paste the entire
+hash string outputted by the program into the `password` field.
+
+Now, Dovecot needs a file describing user accounts on two separate occasions:
+* To check whether a client logging into the IMAP server
+  is valid and has given the right password.
+  This is handled by the `passdb` block(s) in the configuration.
+* To know which email addresses the server is responsible for.
+  This is given by the `userdb` block(s) in the configuration.
+
+These functions aren't necessarily fulfilled by the same `users` file:
+you can map multiple email addresses to one acccount,
+or multiple accounts to one email address.
+For simplicity, though, we'll use the `users` file for both:
+```sh
+passdb {
+	driver = passwd-file
+	args = scheme=ARGON2ID-CRYPT username_format=%n /etc/dovecot/users
+	#args = scheme=SHA512-CRYPT username_format=%n /etc/dovecot/users
+}
+
+userdb {
+	driver = passwd-file
+	args = username_format=%n /etc/dovecot/users
+	override_fields = uid=vmail gid=vmail home=/home/vmail/%n
+}
+```
+The `driver` option sets the kind of table Dovecot should expect.
+We tell it to use a file in the `passwd`-like format described above,
+but other possibilities include e.g. an SQL database.
+The options available in `args` depend on the chosen `driver`.
+
+In the `passdb` block, the hashing algorithm is given by `scheme`,