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