Since over a week, any incoming emails my server receive are trapped in the deferred queue and not send to the recipient.

I'm not sure what's going on. It used to work flawlesly. The only thing that I can think of is that I changed ISP around that time but I don't know why or how it could have affected my mail server.

I have no alarming messages logged in the /var/log/messages and the messages in /opt/zimbra/log/zimbra.log seems fine. Only one line stating something about pop ERR.

pop - -ERR inknown command (AUTH )

I know it's not much to go on but can you at least direct me to where I should go to troubleshoot this problem?