Hi,
Thanks for posting your question in the Microsoft Q&A forum.
Based on your description, I conducted related experiments to invite external people in the Microsoft Entra Center.
After assigning the M365 license, I don't see that external mailbox in Exchange Online.
I tried to log in using my external email account and password, but an error occurred.
So it is not possible to log in to Outlook by assigning a license to external users.