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Access mailbox as an external user
How can a user from Tenant A access their mailbox in Tenant B? My current situation: I have set up B2B collaboration in Microsoft Entra External ID I invited a user from Tenant A to Tenant B ...
- You cannot, this is not a supported scenario.
VasilMichev, thank you for your quick reply. 
It's a shame that doesn't work. Why can I create a mailbox for the external user and give this user access to shared mailboxes if he can't use them at all?
Do you have any idea if this will be supported at some point?
Since the topic of multitenant organizations is still very new in Microsoft 365, I hope that this scenario will be supported at some point. Not much is missing anymore, just the login. 
MTO is pretty much only supported by Teams at the moment, whether other workloads will follow suit remains to be seen. Some scenarios are actually possible for B2B Guests, they can be assigned as group owners or even admins. Mailbox was also an option back in the day, but they patched it since: https://www.michev.info/blog/post/2256/some-new-interesting-experiences-with-guest-users-in-office-365