Not sure what's gone wrong exactly but I have a user that Guest's into our environment who isn't receiving any emails when part of a Team.
Pretty sure this has been caused due to them not having a GuestMailUser contact in exchange, just MailUser. I've fixed this before by deleting and restoring the account (this generates the GuestMailUser contact) but this hasn't worked and has given them a bad UPN.
Hi, if you're talking about the group mailbox settings that decide if one's a subscriber of email and/or events you should check the UnifiedGroupLink settings. Guest users always gets added to that list to get notifications and would explain why your "workaround" by deleting the account has worked. Can't say why they don't receive them anymore though. But start with the below settings and perhaps both org. need to run a message trace as well.