How can I forward mail sent to <user> to their external Teams guest email w/o giving them a paid mailbox?

GraniteStateColin 146 Reputation points

In Exchange Server, it was trivial to set up a user for mail forwarding. If our domain were, just create a Mail Contact, say:, don't assign them a license or mailbox, but just set the account for forwarding.

In Exchange Online, if that user's external mail address is already registered in Exchange as a guest user for Teams access, then it rejects trying to add the user as a Mail Contact because it says the email address is already in use.

I have searched and tried various combinations and have not found anything that works. The closest recommendations appear to be to create a mailbox for the user, but that requires a Microsoft 365 full license, which is precisely what I'm trying to avoid. I only want to pay for their Teams license. Because the free Mail Contact would work absent Teams, that seems fair: The only thing that changed is adding Teams for those users, so we should only have to pay for their Teams license.

Note that I have turned on Anti-spam Outbound Policy -> Automatic Forwarding Rules -> On (forwarding is enabled).

Is that something that MS has worked to prevent, so if we want to give someone the benefits of being an internal user with Teams access, we have to pay for a full license if we want them to get mail at our domain? We can't afford to do that.

Is there any way to forward messages that come to to an external address (and these are mostly personal email addresses, like NOT on another MS tenant), and also have that same external address be used for signing-in to the company as a guest for Teams access without paying for a full mailbox and mail license?

Microsoft Exchange Online Management
Microsoft Exchange Online Management
Microsoft Exchange Online: A Microsoft email and calendaring hosted service.Management: The act or process of organizing, handling, directing or controlling something.
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  1. Andy David - MVP 142.7K Reputation points MVP