There's no reason to accept more than four concurrent ESMTP
connections from the same IP address. Reduce MAXPERC to 10, and
MAXPERIP to 4.
Oh, yes there is. It's called NAT.
The problem is particularly acute if you've got people reading their
mail using Mozilla, as it seems to like to open multiple concurrent
connections to the IMAP server. Two people behind the same NAT gateway
reading their mail with Mozilla at the same time can quite easily
exhaust a four connection limit.