We are using Exchange 2010 and Sophos.

I've got one email that keeps trying to send, but gets this error:

2015-10-07 17:40:41 name@server.com R=dnslookup defer (-1): host lookup did not complete

In the Sophos web interface the email is in SMTP Pool and is status in the Spool is Waiting.

What should I do to diagnose this problem, and then how can I resolve the problem and send this email?

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