At first I was using 'example.com' as my MX record and everything worked fine, IMAP, SMTP, sending mails from the mail clients. And then I changed my mx record to 'mail.example.com' and everything turned into mess.
When I reconfigured the mail clients to use 'mail.example.com' again in server settings, it didn't work, probably some wrongdoings in postfix, but due to my deadline I decided to cut time and revert the MX record back to 'example.com'.
I am using iRedMail (Dovecot/Postfix) as mail server. I have the roundcube as my main webmail and rainloop as my second webmail. I am doing fine with the iRedmail main roundcube sending emails from webmail but I still have these error on other webmail clients nor desktop/android mail clients