Resolve issue - Shared mailboxes showing non-accepted domain addresses when associated to AD objects
I have come across an issue where non-Accepted Domains are being listed as valid email addresses for Shared mailboxes. This affects mail flow during migration batches when using 3rd party tools.
The scenario is encountered when the following is in place:
• DirSync is configured to replicate AD objects to your Office 365 tenant
• You license an Office 365 synced account with an Exchange Online license, convert the mailbox to Shared, and then remove the Exchange Online license.
• For Shared mailboxes which include Non-Accepted Domains - license the associated account in Office 365 and after a few minutes the Non-Accepted Domains will be removed. You can then remove the associated license and the Non-Accepted Domains do not return, even after a Directory Sync.
• NOTE – Any attribute changes to the associated AD object which are subsequently replicated to Office 365 will return the erroneous proxy addresses.
There is an issue in the workflow, as you shouldn't be able to assign non-accepted domains to a Shared mailbox.
This will affects a large amount of migrations when employing 3rd party migration tools which require a mailbox OnPrem and in Exchange Online at the same time.