Issue: If a user was imported via AD but shows up with the wrong username on the O365 side (ending in .onmicrosoft.com)
Resolution:
1. Go to Active Directory Users and Computers
2. Go to View and select Advanced Features
3. Go to properties of user account
4. Go to Attribute Editor
5. Edit Proxy Address to: SMTP:user@domain.com
6. Open PowerShell as Admin
7. Connect to the online service with the following commands:
Install-Module MSOnline
Import-Module MSOnline
Get-Module
Connect-MSOLService
8. Now fill in the username for the administrator for O365.
Set-MsolUserPrincipalName -UserPrincipalName user@company.onmicrosoft.com -NewUserPrincipalName user@domain.com
Changes should take effect within a few moments and set this user as the default username