Someone asked me the other day about how to get additional UPN suffixes listed in the UPN suffix dropdown in New Mailbox GUI wizard. My initial instinct was the obvious: Add some additional AD domains to your forest and they'll show up.
Of course, this is not what this person was really asking... they wanted to know how to add "alternative UPN" suffixes (for domains that do not exist in the AD forest).
Good news is that there is support for this scenario in Exchange 2007! All you have to do is add the alternative UPNs to your AD and they'll automatically show up in the Exchange 2007 GUI wizards where appropriate.
Here are the steps I tested to get this working:
1) Open up AD Domains and trusts (domain.msc)
2) Right click on the root note and select properties – you’ll get this:
3) Add/remove alternative suffixes here to your hearts desire. Then close out of the dialog.
4) Back inside the Exchange console GUI you will now see this in New Mailbox wizard: