Custom Mail rejection message in mail flow rule

Hi,

I have a rule that blocks everyone who emails group A EXCEPT members of group A.

(only members of group A can email another member in group A).

my custom blocking message is  "this user does not exist" 

and it works if the emailer comes from the same AD.

however, i have another group of users that is synced over from another AD via FIM, 

their blocking message became the default one which is error code 5.7.1

please advise how i can set FIM users to receive the same "this user does not exist"  error message.

Thank you!