I am using:
OS: Debian 5.0 Lenny Virualmin: 3.68 gpl Usermin: 1.400
When I successfully login to Usermin (port :20000) and then Usermin -> Mail -> Read Mail I got an error "An error occurred listing mail in this folder : Failed to connect to localhost:143 : Connection refused" and it's the same for every other account (root, admin domains, user domains).
When I'm using email with programs like Windows Mail (Vista), everything works fine (incoming/outcoming).
I've check /var/log/mail.log and /var/log/dovecot.log and there is nothing about that error, even no connection log when trying to connect by Usermin interface.