I keep having a recurring issue involving mail that users send getting stuck in the mail queue and being delivered over and over until I manually trash the queue. This has happened on multiple servers with Interworx and I have absolutely no idea how to stop it from occurring. 
Has anyone else seen this issue?
Its a problem with the remote SMTP (the person you are sending to). I had this problem and was a real headache until I think Chris gave me the idea to increase my SMTP timeout. The problem is the remote server accepts the mail, but before it sends the ACK back to your SMTP server Qmail timeouts waiting for the ACK and therefore assume an error in tranmission. Qmail then does what it is supposed to do on error which is to leave it in the queue to try again later.
/var/qmail/control/timeoutremote
If this is set to like 10 try bumping it up to 50. If its at 50 try bumping it up to 75 and keep bump’n until the problem goes away.
Actually it was Paul who helped me 
Here is a link to my thread from way back when…
http://interworx.com/forums/showthread.php?t=388