We have a domain called ABC.local. I have a new domain called ABCxx.com. The old domain has an alternative UPN suffix assigned for ABCxx.com. The
old domain has an exchange server with email addresses in the format of user at ABCxx.com. They are authenticating to the domain servers and the exchange
server as ABC\user. What would happen if I removed the alternative UPN from the old domain? Our intent is to migrate from Exchange 2007 on Server 2003 into the new domain to Exchange 2013 on Server 2008 R2 server. They are dead set on using the same email
addresses they have now so the option of just changing that is gone :( I also have to keep email live and running throughout the transition. I have a trust setup but, since the UPN for ABCxx.com exists on the ABC.com domain, I cannot share resources
between them. Suggestions? Thank you to anybody who may have the solution to this mess!
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