I have two Office 365 email accounts that I set up a couple of years ago where messages are delivered to 2+ users (e.g., firstname.lastname@example.org delivered to email@example.com and firstname.lastname@example.org). I was setting up another address today and I cannot figure out how the other two work. I did not set up groups or shares and the messages aren't set up to be forwarded. Yet, we receive our messages just fine.
Where else can I look?