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?
Best Answer
On-premises, this would allow a user account to logon to the network and access resources as would any other user (workstations, file shares, LDAP integrated services, federated services, etc.) - the only difference being that mail would be delivered to a mailbox outside of the Exchange Organization.
Similarly, a mail user in Office 365 / Exchange Online would allow a user to logon to access various hosted resources (albeit not a mailbox, as the mailbox is outside of the tenant). An administrator would be able to grant access to all of the various resources in Office365 (Teams, Sharepoint, Office licenses, etc.) all while maintaining the user's mailbox outside of the tenant. In any event, there is much more in Office 365 that can be accessed than just a mailbox. If you were to attempt to logon to any of the other services in Office 365 with that mail user (assuming they were sufficiently provisioned), I believe you would better understand the purpose.
--
For more information (though outside of the first paragraph, it mainly pertains to how to create and manage them), please review this article; relevant information follows: