RUS

I have 5 domains, domin.grp which is the main internal domain, the othere domains “domins.com”
the problem I have is that when the RUS updates the addresses it changes every user’s primary email address to the the internal one “domain.grp” there fore we cannot recieve emails from the out side, I dont understand why is setting up the internal domain as primary email address, I looked at the default recipient’s policy and it shows there the correct domain set as primary, I’m not sure where to check but every time it updates I have to cahange the primary email address for each user to the correct one, one by one, please advice.
thanks