From your description, I am guessing that you are doing some time
consuming validation on SMTP transactions in your outgoing MTA andOutgoingRunner is hopelessly backlogged. If this is the case, it
should be obvious from a detailed look at Mailman's 'smtp' log. If
you're seeing SMTP for hundreds of recipients taking more than a few
seconds and the difference between the timestamps of adjacent entries
equal to the duration of the first entry, this is your problem.
For starters, don't do DNS validation of RCPT commands in the MTA. Then
look at the tuning tips for your MTA in section 6 of the FAQ
<http://www.python.org/cgi-bin/faqw-mm.py>.
The cause: the qrunner was sticking.
Rebooting the machine after deleting the E-mail that it was sticking on seems
to have fixed the problem.
andy