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 firstname.lastname@example.org 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?