Staged migration O365 Error: The source email address couldn’t be found in the on-premises domain.

Since we all  (including customers) embrace the cloud, I have been working a lot with migrations lately. I often encounter some issues, problems or challenges but most often these have been encountered before and the solution is shared online. However this time it was not.

So this customer had an Exchange 2007 environment and wanted to migrate to Exchange Online. During the staged migration 20% failed with the following error:
EmailAddress,ErrorMessage
<user@mydomain.com> The source email address <user@mydomain.com> couldn’t be found in the on-premises domain.

If you search for this error online several suggestions turn up. All of them were okey

• Verify that the on-premises user’s UPN and Primary SMTP
• Verify DirSync and the MsolUser
• Verify that the e-mail address in the CSV file is the same as the primary SMTP address of the on-premises mailbox
• Verify that the user isn’t hidden in the on-premises address list. If the user is hidden, clear the Hide from Exchange Address lists checkbox on the user’s properties page in Exchange Management Console, and then retry the migration.

Since I was unable to find a solution I called MS support.
After some troubleshooting they provided the following workaround.
Move the faulty mailboxes to a new mailboxdatabase or run the New-MailboxRepairRequest.
But New-MailboxRepairRequest was not a option for me this Cmdlet is not available on Exchange 2007 so I moved a couple of mailboxes to another DB.
This was time consuming so I had to find another way. When you move a mailbox several things happen, one of them is reading and changing attributes.
So the solution was to temporary change the Alias and Primary SMTP on the mailbox.
After that I changed it back to the original value it migrated successfully to the cloud J

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