By default, the first created email address in a mailbox is used as service user's login to the mailbox. You can see the service user's login on the General tab of the Mailbox screen, in the Login field.
If you want to change the service user's login, perform the following steps:
- Open the Email addresses tab.
- Start changing the primary email address. Refer to Changing Primary Email Address.
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While changing the primary email address, select the Synchronize UPN logon with primary email address checkbox. As a result, the new primary email address will be used as service user's login.
If you don't select the Synchronize UPN logon with primary email address check-box, the service user's login will remain the same, though the new primary email address will appear in the From field of email messages sent from the mailbox.