Access mailbox as an external user

[hs_nico]

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
• This user is of type member (not guest)
• This user has the identity ExternalAzureAD
• In Tenant A the user has the Microsoft 365 Business Premium license
• In Tenant B the user has the Exchange Online (Plan 1) license
• There is a mailbox for the user in Tenant B
  • Other users from TenantB can access it
  • The emails from this mailbox can be forwarded
• In Tenant B the user can be added as a member of a shared mailbox

But how can the user access these mailboxes? When I try to add a new account in Outlook and enter the user principal name (Tenant B) I get the following error code: CONNECTFAILED. When I select "Open another mailbox" in Outlook on the web and enter the userprincipal name there, I get this error:
BootResult: accessDenied
err: Microsoft.Exchange.Data.Storage.MailboxUnavailableException
et: ServerError
estack: Microsoft.Mapi.MapiExceptionUnknownUser

Feels like I'm almost there. I “just” have to log in. 

Thank you very much for your help and kind regards
Nico

[#2 · Accepted solution]

You cannot, this is not a supported scenario.

[#3]

@Vasil Michev, 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. 

[#4]

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-3...