All patches done. I do have a lot of users and if I look at the maillog activity, there are people popping their mail every minute and they are all authenticated users. There is a constant flow every second or so.
Sounds like you either have too many users on the one server (how many do you mean by a large number) or have a look at the logs and ask the people checking every minute to only check their mail every 5-15 minutes - most will oblige!
Thanks...