[Exim] authenticators: how to prevent an unlimited number of…

Hi,

When using for example a mysql query with salted/crypted password for
authentication when receiving mail via smtp (see
http://bugs.mysql.com/?id=784), how can I prevent an attacker to have
unlimited number of tries at guessing and verifying the password via exim?

Is there a way that exim could limit for example to 3 wrong passwords per
hour?

Many thanks for any hints in advance!

Ralf