Does your email address policy include mail user/remote mailbox recipient types?
Why does primaryEmailAdress stay same when account last name is changed?

When a person gets lastname changed. Primary SMTP adress stays same. This is only the case with the remote mailboxes.
Automatically update email addresses based on the email address policy is enabled on the account but the emails stay the same. Nothing gets updated under proxyAddresses attribute.
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KyleXu-MSFT 25,866 Reputation points
2021-03-09T02:40:25.497+00:00 I tested it in my lab, the Email Address Policy is created on the Exchange on-premises.
After changing the last name for one remote mailbox, I could see the Email address changed immediately from Exchange on-premises admin center(Old last name is "Ma").
Then I run command below on AAD Connect to force sync local AD with AAD:
Start-ADSyncSyncCycle -PolicyType Delta
After waiting for a while, I would see the new last name show in AAD, the primaryEmailAdress also changed:
So, Make sure the new last name is sync to AAD first, then refresh the Office 365 admin center and check again.
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