As Synology NAS:es uses Roundcube for the “Mailstation” service, it’s rather easy to change desired settings.

To force HTTPS on all connections to Roundcube:

  1. Connect to the Synology NAS through SSH

  2. Edit the file: /volume1/@appstore/MailStation/roundcubemail/config/main.inc.php

  3. Under section SYSTEM (around line 306)
    Change $rcmail_config[‘force_https’] = false; to true

  4. Save and quit.