External Program for Authentication?

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)