[Exim] exim rejecting form mail

This is a multi-part message in MIME format.
--
[ Picked text/plain from multipart/alternative ]
I have a client who subscribes to an email list which is distributed
through a script called AspMail. The client is complaining that only
some of the email gets through to her and that the rest of the time it
gets sent back to the mailer( AspMail ).

How can I tell exim not to reject this host?


Thanks in advance
Tony v.
--