[Serusers] SER and interdomain routing

Hello, has anyone any pointers on how to implement interdomain routing with SER _without_ using the domain part of an uri when dialing. Example: I want to map sip:400000@mydomain (not only one id but a complete range) to sip:400000@otherdomain. "mydomain" is served via my Proxy server and otherdomain by someone else. My first thought was to just rewrite the "to" part of the uri but AFAIK thats not suggested because it violates the SIP RFC. On the other hand i dont see how to circumvent rewriting the "to" uri unless both sides use E164/ENUM Numbers or the enduser uses complete sip addresses and not only the (numeric) user part. best regards, Arnd