The following article is intended for qmail server administrators and postmasters of anykind that need help with managing a qmail system.
If you use qmail as your Mail Transfer Agent (MTA), then you need to make sure your queue does not get clogged up with either inbound or outbound emails.
Supposing your mail is stuck in your queue, once you have looked over your qmail queue, you can determine how to fix it (or at least it should point you in a useful direction).
There are many commands and utilities such as qmailctl and qmHandle which you should have installed in order to have your server do lots of backend dirty work, managing your queue effectively. They are also great analysys and repair tools. Check them out once you've determined if your queue is giving you any problems.