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I have a user who is trying to send meeting invites to several external folks, and he's getting a "Delivery Status Notification (Delay)" message.

On the exchange queue, I see the message "Unable to open the message for delivery"

I had the user send a meeting invite to my gmail account and it came in just fine. Any ideas what the issue is?

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Perhaps the external user's mail server can't be reached? The delivery delay message just tells you that the Exchange server has tried to connect to the recipient's mail server for (usually) a few hours and hasn't yet been able to send the message. It may have nothing to do with the fact that it's a meeting invite.

You could try sending an email to the recipient to rule that out.

The server by default will keep trying to send the message for 24 hours (I think that's the default) and then return a delivery failure message to the sender.

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