Can a guest account (different O365 tenant), have a mailbox/SMTP address in another tenant?

VasilMichev, thanks for that article reference. Very interesting indeed... As noted, the goal was more focused on allowing us to trust those third parties to manage their identity. We've had one or two occasions where a third-party staff member was terminated but we were not informed for days or weeks. If we'd been using their true tenant user account as a guest, they'd have been disabled as soon as their company disabled them (yes, I realize there could still be some risk here). I'm interested to see how MS continues to develop their guest functionality as it relates to O365. Thanks for the quick response!