Here is my scenario:
We have a Domain Trust between 2 organisations who recently merged, let me call them A.local & B.local.
Both domains have exchange 2010, I've configured cross forest availability between them so calendar sharing is possible. I used FIM (Forefront Identity Manager) to sync users of domain A.local as cross forest contacts in B.local. Users from A.local can look up calendars of users from B.local and the other way around, everything just fine by now.
But now the two organisations want to share the same SMTP addresses over the 2 domains. I've configured this by changing the accepted domain as an internal relay domain, so mail for A.local arrives at exchange and delivers it to B.local if the mailbox doens't exist on A.local (through send connector). Mail flow is working fine in this situation.
The problem is the exchange availability (free/busy) between the two domains:
If a user of A.local with primary smtp address userA@ABC.COM want to lookup a calendar of a user in B.local with primary address userB@ABC.COM this doenst work. MX records of ABC.COM are pointing to A.LOCAL. It is working the other way around. An autodiscover record for domain ABC.COM is created an pointing to A.LOCAL.
If you need more info, shoot ¿
Any help appreciated!!