Question : Qmail Queue Filling up with NDR's

My qmail server will occasionally get bloated with outbound messages.  A signal that this is happening is that when I send a message to "[email protected]" I will get an NDR from qmail saying that the message I sent to "[email protected]" could not be delivered.  It's the exact message I sent to "[email protected]" and I did not include sue in the cc: or bcc: or anything like that.  I don't even know sue, nor do I have her in my contacts, etc etc.

I will get several of these NDR's for several addresses that I didn't put on the original email.  Then the queue bloats to huge numbers (thousands or tens of thousands) and legitimate outbound email is delayed for long periods of time, causing trouble for the business.

It's running on a BSD server.

How can I, a qmail novice, find out more about the root cause of this problem and put a stop to it?

Thanks for your time.
Scott

Answer : Qmail Queue Filling up with NDR's

Thanks for the contributing comments.

I discovered that although we get all incoming mail via a 3rd party spam filter server, we were accepting messages from anyone on the internet.  I edited tcp.smtp in the /home/vpop/etc dir to disallow any connections not from our service provider.

Mail was still not going out, but the logs were MUCH easier to read now.  They recorded DNS errors, so I looked a little deeper and found the DNS this machine was looking at was not ours, but somewhere else on the Internet, and has apparently been taken offline.  I changed it to look to our internal DNS server and all is well.