In the Exchange Admin Center (https://outlook.office365.com/ecp), regular users are created in the "Mailboxes" tab. External contacts for mail forwarding are created in the "Contacts" tab with the type "Mail Contact".
On the "Contacts" tab, when creating a new entry, there is an option to create a "Mail User" instead of a "Mail Contact". As far as I can tell, the only difference between them is that a "Mail User" also has a password field:
Based on that, I thought that a "Mail User" could be used for things like scanners or other devices, or "service" accounts such as those that would be used by a support ticket system.
However, the Microsoft documentation for setting up a scanner or similar device says this:
You must have a licensed Office 365 mailbox to send email from.
Additionally, trying to send mail as a "Mail User" account (using
swaks) gives an error:
550 5.2.1 Mailbox cannot be accessed [BL0PR0102CA0003.prod.exchangelabs.com]
Given that this account seemingly cannot actually be used to send mail, what is the point of it?