I just added a new user account in ADUC. After creation I went back to the General page and filled in the correct email address. After it synced with O365 the user appeared. I assigned a license and an Exchange Online email box was created. So far so good.
I want to add another email address to that account, though. When I tried in O365 it popped up an error and told me to do it on my local Exchange Server. I didn't create an Exchange account locally so can't do that. My local Exchange Server will be going away fairly soon anyway, so that's obviously not the way I want to handle this in the future.
So... how do I add these extra accounts?