I know this has been discussed in other threads but I'm trying to find a clear answer on how/if this can be done. I've got a customer who has to co-exist between GroupWise 8 and Exchange 2010 for some period of time. Some users will be migrated from GroupWise to Exchange but the period of coexistence will likely be somewhat long.

Our goal is to be able to share the same namespace (client.com) and setup SMTP coexistence between the two organizations. Actually getting SMTP coexistence working is very simple and I have that working right now. I've also got an external domain configured in GroupWise with external contacts, and I can pick those contacts from the address book and email them without issue.

The problem I have is when I need to migrate a mailbox to Exchange. Using the "old" method of the Exchange 2003 connector and API Gateway in GroupWise, I could simply set the visibility to None on the migrated mailbox and then create a new external user with the same email address and everything works. However for whatever reason when I try to do the same with the external user it complains that the address is already in use.

If I simply define a different email address (last.first@client.com instead of first.last@client.com) then mail routing works properly. But I'd really like to keep the same email address to maintain consistency with external email recipients.

I hope this makes sense. Does anyone have any thoughts on this?