I was looking through the mailman docs and came across this.

Maximum number of messages your list gets in an hour. (Yes, bounce
detection finds this info useful) (Details)

Could this be set too low perhaps?  Maybe when the list was smaller or
something.

-- 
Bryce Chidester
Network Administrator
CoBryce Communications
Bryce AT BryceCo DOT Net
http://www.bryceco.net