This thread is quite old however I thought somebody might find this piece of advice helpful enough.

On my postfix setup I make sure every failed login attempt is followed by a delay (i.e. 1 second) and then every n-th failed login is followed by longer delay (i.e. 10 seconds). I'm still probed however attacks take less cpu and attacker realises soon enough there is no possible way to crack anything when each attempt is delayed.