We recently performed a mock cutover migration for a client from Exchange 2007 to Exchange Online and ran across a very odd issue. Following the guidelines regarding cutover migrations on TechNetwe had configured OWA and verified remote connectivity. When we went to verify the domain it let the client go through the process annotated in this video to add the domain but would not successfully verify on the last step.
As we had already verified remote connectivity and setup the migration batch this was a bit disconcerting. After the client removed the domain from the Manage Domains page in Office 365 we were no longer able to get past the first step of Adding a Domain. We attempted to use PowerShell to “force” the issue which initially appeared to work but unfortunately when we got to the last step we received the same issue… Sorry, you can’t add “domainName.com” here because it’s already in use.
As this was simply a mock migration we hung it up for the night after about an hour of troubleshooting and in the morning the client contacted Microsoft regarding the issue. It turns out that someone in their organization had done a trial of Office 365 a few years ago and the domain was still considered in use on that trial subscription. Once Microsoft removed the domain from the trial subscription we were good to go for our mock migration.
Hopefully this helps someone out in the future as we were in full “bang head on desk” mode for a while but ultimately edge case situations like this are the exact reason we do mock migrations whenever possible.