NW65sp4 + bm38sp4

Lets say I manage the domain test.com. When someone at the public side of my
mail proxy sends a message using as sender something like xyz@test.com (
spammers trying to behave like internal users ), bordermanager ( before
upgrade - 3.6 ) used to reject this message. Now ( bm3.8 ), I dont know why,
it receives and relay the message to internal smtp server. How can I avoid
this situation ?