Exchange 2010 recipient address rewritten

Hello,

I have a problem with Exchange 2010.
I have 2 email domain where mx is hosted on a linux server (spam/antivirus gateway)
This linux server send mail to my exchange after filtering (directly to port 25)
Both domain are accepted on exchange and email are deliverd well.

The only problem I have is that exchange "rewrite" the recipient adress with the default domain of user AD.

Here is the explanation:
I have a user1 mailbox wich have 2 domain alias
user1@domain.com (default in AD)
user1@domain.lu

If I send mail to user1@domain.com => mail is received and I can see in Outlook Web app that the reipient is user1@domain.com, so it's OK (the AD defaulkt adress is user@domain.com)

If I send a mail to user1@domain.lu => mail is received and I can see in Outlook app that the recipient is user1@domain.com. This mean exchange always rewrite the recipient adress with the default domain! So this is a problem for my user because they cannot "see" when someone send mail to them with domain.lu domain...

So the simple question is, how to tell exchange to not rewrite the recipient adress and keep  the original???

Thanks for help