Config :
BM 3.8 on NW5.1 SP6
Internal SMTP server that relays through NAT
(not using BM outgoing relay)

My Problem :
A user cannot send a message to a collegue (same
domain name) : SMTP error 554 on MAIL FROM.
I think this is due to the fix #2 in TID 10080936:
"Does not allow mail relay from Mail Proxy primary domain
(impersonating domain). It
checks for the IP address that is generating e-mail and compares it to
the internal mail
server. If they are different the proxy errors out with <554
transaction failed>"

Is there any way to bypass or to config this feature?
If not how could I config my system to allow to send
an email from to