I need to understand Exchange online in hybrid environment a bit better, so the question I have is if for example I have a hybrid environment with some mailboxes online and some on premise. If on premise Exchange becomes unavailable and the mailbox is hosted on premise, what will happen to an email sent to that mailbox from outside the organizational. MX record is pointing to the Exchange online.
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If On-premise Exchange server is working fine, the message from external will deliver to Exchange Online (bases on MX record), Exchange online will route message to On-premise Edge or CAS server, then deliver it to proper mailbox.
If On-premise Exchange server is down, the message will be stuck and returns an NDR to sender.
Jianfei Wang
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