I recently ran into an issue with a customer that took me a while to get a solution for. While doing a hybrid migration from Exchange 2003 to Exchange Online, I had one user whose email alias showed up as alias3834@domain.com instead of the expected alias of alias@domain.com  in the Exchange Online tenant.

The user’s alias was set correctly in the on-premises Active Directory, and no changes we made to AD would correct the users alias in Exchange Online. I believe that this was caused because this user setup his first administrator account in his tenant with the same alias as his on-premises email account. The solution for this issue is as follows

1. Download Windows Azure Active Directory Module for PowerShell
2. Connect to remote PowerShell for your tenant
3. Run Set-MsolUserPrincipalName –UserPrincipalName alias3834@domain.com –NewUserPrincipalName alias@domain.com

…and you’re good to go.