Do you have a valid SSL certificate set up for email?
I have a suspicion that this is due to the default Kloxo certificate being self-signed.
Roundcube sends on my Centos 7 install of KloxoNG. I have /var/qmail/control/servercert.pem symlinked to an Letsencrypt certificate in /home/kloxo/ssl/ that is setup up for the domain of the mail server .
eg If mail server domain is mail.mydomain.com, set up a certificate and make a symlink servercert.pem pointing to /home/kloxo/ssl/mail.mydomain.com.pem .
I will have to test more to see if this is actually the problem.
- This reply was modified 3 months, 2 weeks ago by John Pierce.