limit pop3 logins to (x logins per n minutes) [qmail-pop3d and vchkpw]

Hello,

is there any "easy" way to limit users' pop3 logins to a maximum
logins/time?

Example:
user john@example.com logs in every 2 minutes - he should be only able to
log in every 5 minutes (or 4.5 so her can log in every 5 minutes for sure).
Ideal solution would be a counter so that he might check his mailbox quickly
bute not too often (some kind of tarpitting the pop3 connection for example
would work as well for me).

I am using a heavily patched qmail system with vpopmail/vchkpw
authentication. One perhaps could write/read some timestamp to the user's
maildir - so I just wonder if there is any patch out there for this problem.

Best regards
Thomas.