would anyone be able to confirm what happens if an exchange on-premise is offline (e.g. for 2 days) with an hybrid environment. Is there any email continuity where the Office 365 would be holding emails that can't be sent to mailboxes on premise, or would the sender (internal on O365 or external) get bounce back message straight away?
Hybrid is not an email continuity solution, if the On-Prem server is offline and messages are directed there, the usual NDR will be sent after the expiry period (which depends on the sending server, so it might as well be immediate).
