We seem to be having big issues with e-mail going out from our exchange 2010 server...
Quite alot in the outgoing queue we're getting 451 4.4.0 DNS query failed
for example i just e-mailed someone at virtualvoiceservices.co.uk and it gave the above.
and it came back immediatly resolving the MX record....
I have set exchange to use external DNS for the external lookups....and ive put the google DNS server to do the lookups 188.8.131.52