I thing best option for this is reinstall server and install iRedMail or reinstall server and install and configure dovecot and postfix manually. Second is not easiest, but I thing it is best way to have best controll about your mail.

And i thing you could fix your issue by forcing different password encrypting method.

I solved it by forcing clients to DONT use any SSL and security things. But this worked for few days then somehow mail server totaly crashed and couldnt work that is another issue and I am now trying iRedMail that i thing is better. Antispam antivirus, Control Panel and all the other stuff included in one installer.