[Dovecot] Unresolved client rejection

[Greg Pearson]

Hi,
would it be possible to prevent users who try to login from an 
unresolved IP from connecting to Dovecot IMAP?
I have a suspicion that it should be done via the authentication module 
(ie PAM) but I am not sure (if this is the case could someone recommend 
a good resource or even a solution).
For example sendmail has a relevant macro to achieve this, I was 
wondering if Dovecot can have similar behavior.
Thanks