dictionary attack defense

[Cliff Hayes]

Does dovecot have any dictionary attack defenses yet?
In the past I have had to implement defense from outside dovecot, but 
since dovecot is at the front lines and therefore is the first to know 
I'm hoping by now there is something we can set.  For example, a limit 
on access failures per minut/hour/day or some such.  If not why not?