[Exim] Recipient Verification against Usernames in a MySQL d…

This is a multi-part message in MIME format.
--
[ Picked text/plain from multipart/alternative ]
This might be a simple question for most of you but I have searched the FAQ and mailing list archives to no avail. I'll admit I'm rather new to exim and the ACL's and its probably due to my lack of knowledge that I am unable to figure this out for myself.

Having said that what I want to do is to check incoming emails to see if they are addressed to valid recipients on our server. Valid recipients usernames are stored in a mysql database. If emails are being addressed to non-existent users I want exim to respond with a simple "No such user here" message and not accept the message. If the recipient exists, I would like mail delivery to continue as normal.

Any help or links to ACL code samples would be much appreciated.

Thanks for your time!

Regards,
Fahd
--