I have a few local groups in AD and one of them required an alias legacy email address adding. You will notice that any groups the a sync’d with Office 365 cannot be edited within the Office 365 portal.
The solution to this is to edit the Properties of the Group and then within the Attribute Editor, look for “proxyAddress”.
You should see the “mail” attribute already populated with the email address in use for that group. “proxyAddress” will most likely be blank however.
Edit this and add:
The key here is that the capital “SMTP” represents the main email address, the rest will all be alias addresses for that group with a lowercase “smtp”.
The same is also possible for any users within your organisation, this came in to play when a user got married and changed their surname, but we kept the previous email is use also.