Alias emails in O365 with hybird on-prem AD sync
If you use O365 in hybird mode, with your tenant sync-ed to your on-prem AD or Exchange server, then you will definitely run into an issue if you try to add an alias email address to a user.
When you attempt to add an alias, or alternate, email in your Exchange Admin Center portal you will see this error message.
To get around this you’ll need to edit the user “local” from your on-prem AD. In AD, right-click and open the users’ properties. Select the tab “Attribute Editor”
You will want to look for and edit the following two attributes.
Add the user’s alias/alternate email address into the above mentioned attributes in the form of: smtp:email@example.com
That’s it. Now you just need to let your AD sync back up to the O365 cloud.
WARNING: If you add it in CAPS (SMTP:firstname.lastname@example.org) then it will get interpreted as the default address and not as an alias/alternate email. Make sure that “smtp” is lowercase.