OK, the idea is that if you are using DirSync for user objects on office 365, there is no way to change the SMTP address on office 365 portal. You can't even do it on the exchange management console, and you'll get almost the same luck if you try using Remote Powershell to O365 or even Exchange Management shell.
After hunting all around the community forums, I finally found a few good way we can do this via powershell (after all, you don't want to go mess with ADSIEdit if you're in an enterprise environment, having hundreds of accounts to provision or modify each day).
Simple method 1 (only set Primary Email - SMTP):
Using Remote PS, run the following command
Set-Mailbox user -WindowsEmailAddress user@domain.com
This is pretty good, as it just sets the primary email to what you want it to be, and forget about everything else. But if you are one who have exchange on premises, the "ProxyAddressess" field in your on premise, federating AD will overwrite that. So this is the only way to modify it:
Get-ADUser $sAMAccountName -properties ProxyAddresses | %{$_.ProxyAddresses += @('SMTP:primary@domain.com', 'smtp:secondary@domain.com'); Set-Aduser -Instance $_ }
For more complicated requirements, you may have to parse through the ProxyAddresses array to remove SMTP fields you don't want, add in smtp fields you want, etc. Maybe if I have some extra time in the future, I shall write a little helper script to do just that. I may even have a need to write such a script in the future, who knows ;-)
