So we have an on-premise AD running, this is connected to my Office 365 (and therefore Azure AD) using the Azure AD Connect. Great.
My only problem so far has been when I needed to add an alias email, normally I would simply log onto the Office 365 Admin Portal, go to my user click edit under the username/email section and add the Alias in there, however after connecting up the AD Connect it now shows;
So how to get around this, it must be done in the On Premise AD, but initially it isn’t obvious.
First, log onto your local AD, then go to Active Directory Users & Computers MMC. From there, click the View along the top and enable Advanced Features.
Now close the MMC and go back in, find your user and go into the properties, you should now see a new tab called Attribute Editor. In there you will find an attribute called proxyAddresses. This is what we need.
You need to add all the addresses you want in here; prefixed with smtp:
Your primary address should start with SMTP: (yes uppercase!), all the others should be lowercase.
Once added let AD Sync and viola! It works.