HOWTO upgrade to Dovecot 2.4 and Ubuntu 25.10

When upgrading from Ubuntu 25.04 to 25.10, Dovecot was upgraded from 2.3 to 2.4.2. Dovecot changed its configuration format.

As there is no migration tool, three files had to be updated manually.

This setup includes:

• mailboxes in maildir format
• standard users (in /home)
• virtual users (in /var/mail/vhosts)
• certificates generated by Let’s Encrypt

These are the changes I made:

10-mail.conf

old

mail_location = maildir:/home/%u/Maildir

new

mail_driver = maildir
mail_home = /home/%{user|username}
mail_path = %{home}/Maildir

10-ssl.conf

old

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

new

ssl_server_cert_file = /etc/letsencrypt/live/mydomain.com/fullchain.pem
ssl_server_key_file = /etc/letsencrypt/live/mydomain.com/privkey.pem

auth-passwdfile.conf.ext

old

passdb {
  driver = passwd-file
  args = username_format=%n /var/mail/vhosts/%d/shadow
}

userdb {
  driver = passwd-file
  args = username_format=%n /var/mail/vhosts/%d/passwd
}

new

passdb passwd-file {
  auth_username_format = %{user|username}
  passwd_file_path = /var/mail/vhosts/%{user|domain}/shadow
}

userdb passwd-file {
  auth_username_format = %{user|username}
  passwd_file_path = /var/mail/vhosts/%{user|domain}/passwd
}