Issue with calendar invites and routing domains


I have an issue I don't quite understand I hope someone can point me in the right direction.

We are in the middle of a merger and party A has GroupWise 2012 and party B has Exchange 2013.
I have successfully setup mail flow between the two systems using subdomains.

Both domains share an address space of and all users have an alias for GroupWise and Exchange
When a GroupWise users sends an mail to an Exchange User the address is found in a Shared Address book.
Thus a mail from for is send to when he replies. matches a contact with an target address of
Email works fine back and forth.

If we do the same with an calendar invite the messages arrives in Exchange but when the uses accepts the invite the “accepted” message never reaches the mailbox of the GroupWise user.
We see it leaves Exchange and we see in the GWIA log and then it’s gone … no NDR nothing.

Who has an idea where to look? Any help p is highly appreciated.

Ical is active
Flat forwarding is enabled