Azure AD B2B collaboration :- MSA mailbox should not be provisioned for users accepting invitation.
Whenever a guest user is invited, if he/she has a live account, the redemption process is completed after the consent, but if he/she is using a gmail or any other provider, the user account is created in live database.
If this is limited to having a set of claims for a user object I think its fine , but for some reason a mailbox is also getting provisioned in MSA.
So real time experience, I have a gmail id - firstname.lastname@example.org which I was using on gmail and know after getting invited by any tenant, I can use this id on outlook.com to send emails.
The id or the mail address that gets mentioned in sender's tab is in the form of guid.
I have also mentioned the entire scenario here :-