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) ?