Within the support organization at Microsoft we definitely see cases where customers are trying to recover deleted mailboxes. Typically, by the time a customer has contacted us they have tried ev...
Updated Sep 14, 2021
Version 5.0
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Agree with Vasil - I just tested and verified that SMTP soft matching will only work if the ImmutableID is empty. Otherwise you get an error in the event logs on the AAD Connect (DirSync) server that objects have duplicate attributes and the sync fails.
Clearing the ImmutableID can only be done from Azure Active Directory PowerShell, and the command for this is:
Set-MsolUser -UserPrincipalName User@Company.com -ImmutableID ""
Additionally, you don't actually need to modify the UPN to *.OnMicrosoft.com, this is just extra work, since you need to change it back later anyway.