External Program for Authentication?

[Marc Perkel]

Is it possible to run a bash script for authentication where a 0 exit 
code indicates success and a non-zero is failure? What I'm trying to do 
is create a shadow IMAP server that authenticates against a different 
server. That way my server will use the same passwords as an existing 
server.

So what I would need is for dovecot to pass the username and password to 
my script, I attempt to log in remotely and if I succeed I allow access 
on my side. My side will be used to configure black lists and where spam 
is dragged from their side to my side. (I'm a spam filtering company)