Setting up an email server in 2020 with Dovecot and OpenSMTPD: extras

This sequel to my earlier guide discusses extra tips and tricks to extend your email setup. This page will be updated continuously as I come up with ideas.

Last updated 2020-04-29.

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:

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:

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:

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:

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:

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 and RFC 8461, 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:

_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:

_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 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):

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:

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's or Ayke'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.

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.

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:

$ 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:

$ 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:

$ 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:

$ 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.

$ 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:

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:

# 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 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 storage format:

$ 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)

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:

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:

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:

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:

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.


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