I have run into a problem after a fresh install of CentOS 7, Webmin, Vitualmin, Postfix and Dovecot. It seem that the hostname was not configured properly. Here is what I did to fix this problem.
Locate and edit this file:
Make sure that the hostname is set to localhost.localdomain.(YOUR_DOMAIN_NAME)
So it will be like this:
Save and start Dovecot!