[Exim] Exim Authentication via PAM ?

I was wondering if anyone can help me with the following problem:

I am looking to centralize all our passwords using SMB_AUTH PAM modules on
all our Unix Boxes so when a user logins in the unix box authenticates
against an NT4 PDC. This has worked fine, the problem I am having is that
Exim uses the passwd file on Unix to authenticate.
Is there a way of getting Exim to authenticate via PAM to the NT4 PDC ?
I'm a complete Exim novice so any help would be greatly appreciated.

Regards,

Matt Clark
NISS