Typically, you use a connector to route messages from your Office 365 mailboxes to your on-premises users. You might also ...

Typically, you use a connector to route messages from your Office 365 mailboxes to your on-premises users. You might also use a connector to route messages from Office 365 to a partner organization. When Office 365 can't deliver these messages (for example, the connector is incorrectly configured, or there have been networking changes in your on-premises environment), the messages are queued in Office 365. After 48 hours, the messages expire, and are returned to the sender in a non-delivery report (also known as a NDR or bounce message).

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