Changing primary SMTP e-mail address in Hybrid Configuration

Scenario:

• hybrid configuration (MS Ex 2010 and Exchange 365), Windows Azure AD Sync tool
• user created in AD on-premises, license assigned and mailbox automatically created in Office 365
• user's primary SMTP e-mail address is user@domain.onmicrosoft.com, and you can't change it to user@domain.topdomain because it needs to be done in AD on-premises

Resolution:

• it can be done with powershell of course but the easiest method is with gui
• open AD Users and Computers, right click the user, properties
• if the Attribute Editor tab is not visible, go to View>Advanced Features
• in Attribute Editor go to proxyAddresses, click "p" to avoid extensive scrolling
• Value to Add: SMTP:user@domain.topdomain

Synchronize your AD with Office 365 via powershell, eg. : Start-OnlineCoexistenceSync

Update (with Azure AD Connect, May 2017):

• Start-ADSyncSyncCycle -PolicyType Delta  (changes only)
• Start-ADSyncSyncCycle -PolicyType Initial (full update)