I'm attempting a Staged Migration from Exchange 2003 on-prem to Office 365's Exchange Online
and I think I have all of the preparation complete yet the migration batch I've created (just one mailbox) fails with the error "A valid source email address ourdomain.onmicrosoft.com couldn't be found on the target.
I have searched for information on this and most results point to the need for the Exchange Online domain(s) to be added as a UPN on our Exchange 2003 AD and SMTP addresses for the Exchange Online to be added to the users that are being migrated. I've done this and it still fails with the same error.
I had a suggestion by Alan Hardisty that maybe our domain hasn't been set as the default domain in O365.
I've checked and it is, but the status says "setup in progress" (screenshot below). When I click "complete setup" and go through the steps, the only thing remaining is to add the MX record for O365. Surely I don't want to do this until the staged migration is complete right?
Am I missing an important step in my on-prem preparation or in the migration batch creation?