A mail contact is an email address outside your organization that you want to be able to add to distribution lists and include in your global address book:
The contacts page
We discussed the difference between mail contact and guest users in Chapter 1, The Office 365 Administration Portal. A guest mail user is a guest user whose email is recorded in Exchange as if they were a mail contact. You cannot create or edit a guest mail user in Exchange; they appear here more for information than anything else. If you need to edit them, use the Azure AD portal.
You can create a mail contact or a mail user here. A mail contact is essentially ...
