This one puzzled me for a while because it wasn’t immediately obvious and I remember there was always a convoluted way of doing things on El Capitan.

The good news is it’s much easier under Sierra and High Sierra. All you need to do is this:

  1. in MacOS Mail, select Mail » Preferences,
  2. select the Accounts icon along the top of the panel,
  3. select your Exchange account from the list on the left,
  4. make sure Account Information is the selected tab on the right.
  5. You should now see an Email Address dropdown box on the right and it will most probably have your Exchange master email address in it,
  6. if you click that dropdown, you should be able to select Edit Email Addresses at the bottom, which will bring up a new panel.
  7. In that panel, just click the + and - symbols to add new email aliases.
  8. You can then select those aliases as the sender address whenever you create, reply to or forward an email.

It has taken a long time but Apple computers now seem to have good support for Microsoft Exchange.