Office 365 alias to email

johnbentrim 

If you are synchronising AD to O365 with Azure AD connect, then you will need to remove the alias on-premise and then either wait for AD sync to run, or force a sync manually. If cloud only, it's a lot quicker and simpler.

Even taking AD sync into account, the process can be done very quickly.  Assuming you have directory sync in place, I'd suggest;

1). Remove the alias from the on-prem object

2). Run manual sync of AD connect from the AADC server by going to PowerShell and running Start-AdSyncSyncCycle -PolicyType delta

3). Check that the alias is removed from the O365 account

4). Create new remote mailbox on-prem and then re-run the manual sync from step 2 again.

You should not see any emails bouncing during this time if you carry out these steps swiftly enough.

If you are cloud only, the process will be even quicker and easier as you can do all of the steps from the Office 365 Admin Center.