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:
Connect to the Synology NAS through SSH
Edit the file: /volume1/@appstore/MailStation/roundcubemail/config/main.inc.php
Under section SYSTEM (around line 306)
Change $rcmail_config[‘force_https’] = false; to true
Save and quit.