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+++ date = '2024-11-19T16:29:01Z' draft = true title = 'How to Set up a Mail Server on Alpine Linux with Postfix and Dovecot' +++

About mail servers

Simply put, a mail server sends and receives email. When ruby@protonmail.com emails klaasje@gmail.com, Protonmail's mail server's MTA sends the email to Google's mail server's MTA. Google's MTA then passes the email on to Google's MDA, which is responsible for storing the email. MDAs also run IMAP or POP3 servers so you can read your emails and send emails with an email client. IMAP and POP3 are protocols for email retrieval over TCP/IP. For sending email, you would use SMTP.

Or visualise it like this:

Ruby's email client via SMTP -> Protonmail's MTA -> Google's MTA -> Google's MDA -> Klaasje's email client via IMAP

A mail server is software which can be run on any computer, including yours. You can host a mail server on a server you already own that is hosting other services, so long as those other services aren't using any of the [mail ports](## Unblock your ports).

Why this tutorial?

There are many tutorials on the internet about how to set up a mail server. I don't claim that mine is particularly better than anyone else's; I'm mostly writing this for my own reference so that I can recreate my own setup on new machines.

There are existing mail server tutorials for Alpine Linux, including ones for Postfix and Dovecot. This tutorial aims to be "monolithic" (i.e. covers the entire mail server setup with all its components), making it easier to follow without trying to follow different tutorials that may not be 100% compatible with each other, and to be adapted to my own specific use-cases, which may not be yours.

I am hugely indebted to LinuxBabe's mail server tutorial for Ubuntu, which is actually what I followed to set up my own server. Essentially, this tutorial could be thought of as an Alpine Linux adaptation of LinuxBabe's tutorial. If you want to set up an Ubuntu or Debian mail server, I recommend following LinuxBabe's tutorial, which is written very clearly and is easy to follow.

The mail server will be composed of the following software:

Component Software
Mail transfer agent Postfix
Mail delivery agent DKIM authentication and signing
Spam filter Amavis SpamAssassin
Antivirus ClamAV
MDA server-side filtering Pigeonhole

Postfix will be our SMTP server, and Dovecot will run an IMAP server for us.

Postfix and Dovecot are required for the minimum of what you'd expect from a working mail server (i.e. a user can log in through a standard SMTP/IMAP/POP3 email client, read their emails, and send emails). The rest is optional and modular, i.e. you can opt to have e.g. Pigeonhole but not Amavis.

We will end up with a small-scale mail server running on Alpine Linux with one domain, and we will use Unix user accounts as mail accounts.

This tutorial was written for Alpine Linux 3.20.3, though will most likely work on other versions too.

This tutorial assumes no prior knowledge about mail servers.

You will need

A server

Any computer will do. This tutorial is for Alpine Linux specifically.

The relevant thing to consider for your server is that port 25 (the port for sending email) is not blocked. Most VPS hosts block port 25 because spammers commonly use VPSes to send spam, so you will need to find one that doesn't block port 25. Some VPS hosts block port 25 by default, but will unblock it upon request, and re-block it if they find you are spamming.

A domain name

Self-explanatory. You need a domain name and the ability to set its DNS records.

Before you start

Set up your DNS records

Firstly, pick a domain for your mail server. If you're sending emails from domain.com, mail.domain.com is a common choice.

MX record

An MX record denotes that your domain is used to send and receive email, and tells other MTAs the domain name of your mail server. We will use mail.domain.com for your MX record. For instance, my MX record looks like:

revsuine.xyz.                    14400 IN MX    0 mail.revsuine.xyz

Mail server records (A, AAAA, and/or CNAME)

Now you need to set a record stating the IP address of your mail server (mail.domain.com above). Depending on your setup, you may want to create a CNAME record pointing to domain.com if the IP address of mail.domain.com is the same as the IP address of domain.com, or an A record if the IP address is not shared with another domain.

I use a CNAME record because the IP addresses of mail.revsuine.xyz and revsuine.xyz are the same, so my record is:

mail.revsuine.xyz.               14400 IN CNAME revsuine.xyz

If you use an A record, your record may look something like

mail.domain.com.                    14400 IN A     ip.address.here

If you use IPv6, you should also add an AAAA record, e.g.:

mail.domain.com.                    14400 IN AAAA  ip:address:here::

A note on my DNS records

I use one server with one IP address for hosting several services under one apex domain. For instance, my static website is hosted at revsuine.xyz; my Nextcloud is hosted at cloud.revsuine.xyz; and my mail server is hosted at mail.revsuine.xyz. To handle this setup, I've created a subdomain master.revsuine.xyz (you can call the subdomain anything you want) with an A record (because your [PTR record](### PTR record) is expected to be an A record, not a CNAME record) to my server's IP address.

I set my server's hostname and PTR record to master.revsuine.xyz. I will also refer to this master.revsuine.xyz subdomain further down when configuring Postfix.

PTR record

A PTR record is used for "reverse DNS", or rDNS, lookup: instead of mapping a domain to an IP address, it maps an IP address to a domain. It is not managed through your DNS manager (e.g. whomever you bought your domain name from), but through whoever gives you your IP address. If you rent a server, be it a VPS or a dedicated server, your hosting provider whom you rent from will be able to manage PTR records. You may have an option to add a PTR record in the dashboard of your hosting provider, or you may have to contact their support to get a PTR record added.

For the above stated reasons, my PTR record for my single server (which hosts all my services) is master.revsuine.xyz.

If your hostname is mail.domain.com, your PTR record should be mail.domain.com.

Unblock your ports

After unblocking ports from your internet provider (e.g. if your VPS host blocks outgoing port 25), make sure the following TCP ports are open on your firewall:

Port Usage
25 SMTP
143 IMAP
465 Email message submission over TLS
587 Email message submission
993 IMAPS (IMAP over TLS)

Postfix

Postfix is a mail transport agent (aka SMTP server). In its own words:

Postfix attempts to be fast, easy to administer, and secure. The outside has a definite Sendmail-ish flavor, but the inside is completely different.

Installing Postfix

On your server, install Postfix with:

# apk add postfix

You likely also want to have Postfix documentation:

# apk add postfix-doc

Verify that Postfix is installed by checking its version:

$ postconf mail_version

Configuring Postfix

Edit /etc/postfix/main.cf.

You should set myhostname to the hostname of your server; [in my case, this is master.revsuine.xyz](### A note on my DNS records).

Now set mydomain to the domain you intend to send email from. For instance, my email addresses are name@revsuine.xyz, so mydomain is set to revsuine.xyz.

myorigin determines the domain name in the From: field of locally sent emails. So you could for instance set this to revsuine.xyz.

maillog_file denotes where Postfix's log file is. By default this is /var/log/messages; you may want to configure Postfix to have a dedicated log file like /var/log/postfix.log.

You probably want to have logrotate rotate your Postfix log. If there isn't already such a file, you want to create one at /etc/logrotate.d/postfix:

/var/log/postfix*.log
/var/log/mail*.log
{
    daily
    missingok
    notifempty
    rotate 7
}

Send your first email

Have the postfix service auto-start upon boot, and start it during this session:

# rc-update add postfix default
# rc-service postfix start

You can now send an email with the following command:

$ echo "test email" | sendmail user@externaldomain.com

Send this email to your email account with an external server, e.g. a gmail account. Note that Protonmail has quite stringent spam filters and this likely would be rejected by Protonmail, i.e. not even reach your spam folder.

Configure email aliases

You can configure aliases for your mail server. Edit the /etc/postfix/aliases1 file.

You shouldn't receive mail as root, so configure root to have an alias to your user, e.g.

root: revsuine

You also must have a MAILER-DAEMON and postmaster alias present:

MAILER-DAEMON: postmaster
postmaster:    root

Note how you can have referential aliases; mail to postmaster is aliased to root, which is aliased to revsuine, so ultimately revsuine will get postmaster's mail.

You can continue to populate the aliases file with whatever aliases you want.

  1. Your aliases file will most likely be in this location by default, but you can run

    $ postconf alias_maps

    to find out where this file should be. ↩︎