The destination email server is offline or unreachable.

Causes for error code ..7
When Exchange Online attempts to deliver a message, the destination email might be unable or unwilling to accept the message. This can result in a temporary _.x.x_ error code from the destination email server (instead of a permanent 5.x.x error code that indicates the message was rejected). Exchange Online repeatedly tries to deliver the message over 2 hours. Only after two days of unsuccessful delivery How to delete archived emails in Gmail attempts do the recipient receive this NDR.

The possible causes of this error are:

The server won't accept delivery of the message.
A network problem is causing message delivery to time out.
Details for error code 5.1.0
The NDR from Exchange Online for this specific error might contain some or all of the following information:
User information section
The server has tried to deliver this message, without success, and has stopped trying. Try sending this message again. If the problem continues, contact your help desk.
Diagnostic information for administrators section
#550 ..7 QUEUE.Expired; message expired ##
The message was considered too old by the rejecting system, either because it remained on that host too long or because the time-to-live value specified by the sender of the message was exceeded.
50 .7.0 Proxy session setup failed on Frontend with '51 ..0 Primary target IP address responded with ... Be sure to record the error that follows this string and the last end point attempted.

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See also
Email non-delivery reports in Exchange Online
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