Dovecot

Dovecot is an open source IMAP and POP3 server for Linux/UNIX-like systems, written primarily with security in mind. Dovecot aims to be a lightweight, fast and easy to set up open source mailserver. The below is for debian 9 (probably also works on buster)

Installation

apt-get install dovecot-common dovecot-imapd dovecot-pop3d

Get SSL certificate

Using Let’s Encrypt, request a certificate for the mail server by (replacing mail.mydomain.com with the fully qualified domain name (FQDN) of your server):

certbot certonly --standalone -d mail.mydomain.com

Configuration

Dovecot’s configuration files are in /etc/dovecot/conf.d/. The default configuration is ok for most systems, but make sure to read through the configuration files to see what options are available.

By default dovecot will try to detect what mail storage system is in use on the system. To use the Maildir format edit /etc/dovecot/conf.d/10-mail.conf to set it:

mail_location = maildir:~/Maildir

To configure it for the SSL certificates, open /etc/dovecot/conf.d/10-ssl.conf and append:

ssl = required
ssl_cert = </etc/letsencrypt/live/mail.mydomain.com/fullchain.pem
ssl_key = </etc/letsencrypt/live/mail.mydomain.com/privkey.pem

To force SSL/TLS encryption, open /etc/dovecot/conf.d/10-auth.conf and make sure:

disable_plaintext_auth = yes

SASL

In the main configuration file /etc/dovecot/conf.d/10-master.conf, uncomment the paragraph for Postfix in the auth service block:

service auth {
  # Postfix smtp-auth
  unix_listener /var/spool/postfix/private/auth {
    mode = 0660
  }
}

This will create the private/auth path for the SASL configuration of Postfix. Because Postfix runs chrooted in /var/spool/postfix, a relative path must be used.

Integration with postfix

In /etc/postfix/master.cf append:

dovecot   unix  -       n       n       -       -       pipe
flags=DRhu user=email:email argv=/usr/lib/dovecot/deliver -f ${sender} -d ${recipient}

Firewall

To access the mail server from another computer, configure the server firewall to allow connections to the server on the necessary ports. The default ports are IMAP - 143; IMAPS - 993; POP3 - 110; and POP3S - 995.