I have a hybrid exchange setup in place. I have created a new user and mistakenly added them with the wrong user logon name within the Exchange admin center (EAC) on-premises. The migration of this mailbox to Microsoft 365 has resulted in a wrong email address e.g mailto:[email protected] rather than mailto:[email protected] I have updated the Active Directory attributes on-prem but it is reverting back to the default. Is this an Exchange change that is required somewhere?
It is because of the default email address policy applied to all mailboxes created in on-prem exchange server. You can uncheck the Automatically update email address based on the email address policy applied to this recipient checkbox on the on-prem mailbox to use the email address policy which will not revert the changes back to what you set. Since you are using a hybrid server and I assume all your accounts/mailboxes are syncing using Azure AD Connect.