I'm using BM3.5 EE with with Proxy version 3.71p

I'm trying to add an extra domain to my mail server (Exchange
5.5), but
BM isn't forwarding incoming e-mails to the new domain. It rejects
them with
a " Undelivered mail : Unknown user "

However when I circumvent the proxy and have the mail delivered
to exchange... no problem.

in the BM access rules I've added the new domain to the same
record that
permits incoming mail on the old domain.

Any one have any idea what could be causing my problem?