My clients will not authenticate either with Outlook via IMAP (even with the SSL settings enabled) unless the cleartext option is enabled.
With cleartext enabled, does it send both the username and password in cleartext? If so, that can't be good. A simple packet sniff could obtain credentials.
Please don't steal old threads for a new problem.
What do the logs say when you auth with SSL?
Mar 30 08:07:11 zimbra postfix/smtpd: initializing the server-side TLS engine
Mar 30 08:07:11 zimbra postfix/smtpd: connect from 69-171-28-162.kntnny.adelphia.net[188.8.131.52]
Mar 30 08:07:11 zimbra postfix/smtpd: NOQUEUE: reject: RCPT from 69-171-28-162.kntnny.adelphia.net[184.108.40.206]: 554
Is there anymore than that?
Also, if you are using an IMAP client, are you telling it to connect via the SSL ports?