I have this problem with outgoing emails on one server. When the queue is clean, it sends emails pretty good, but in a few days, it happens that queue gets filled up with a few MAILER-DAEMON messages that seem to be stuck forever and new regular outgoing emails seem to be waiting for those MAILER-DAEMON messages to be sent. However, those are usually answers to SPAM emailes, therefore they are undeliverable.
What's even worse, flushing the queue doesn't help – it just creates bunch of qmail-remote process trying to deliver "bad" and also "good" emails, but they disappear in a few seconds and nothing gets delivered and the queue is still the same.
Anybody has idea why the messages are stuck in qmail queue and don't get delivered (good messages) or don't get thrown away (undeliverable bad messages) ?
Best Answer
The messages are stuck in the queue because you are accepting and then bouncing spam, rather than rejecting it during the SMTP session. spam bounces like these are often undeliverable because the return email address is bogus or unreachable.
Even when they are deliverable, it's still a Bad Thing. It's known as Backscatter. Some people regard hosts that generate backscatter as little better than actual spammers. It certainly causes the same kind of harm to systems and networks as spam does and can get your MTA's IP address onto various RBLs that list backscatter-sources.
The solution is not to clear them out faster, the solution is to not generate backscatter in the first place. This would be good for you, AND good for everyone else.
Unfortunately, qmail is broken-by-design in this regard, you have to patch or replace parts of qmail in order to bring it up to something approaching modern internet MTA standards.
There's another Q here on Server Fault on this very topic How might I stop BACKSCATTER using Qmail?
You might find it easier and more productive to switch to a modern, well-maintained MTA like Postfix or Exim.