This doesn't seem to be a huge problem as it only seems to be affecting one or 2 external clients. Emails from the users here to this other company get caught in the queue with the Unable to bind to the destination server in DNS as an error. I've had a look on the net and found a few things about removing entries for external DNS servers in the SMTP virtual server (which was empty anyway).
I've run an smtpdiag test, everything has passed barring:
Checking MX records using TCP: grs-group.co.uk.
Warning: The TCP DNS query returned no results.
Checking MX records using UDP: grs-group.co.uk.
Error: No MX or A records were found for the remote domain. Verify that the
remote domain is valid. Your firewall allows outbound DNS queries (Windows
NT/2000 Server requires TCP), and your DNS server can resolve external domains.
Am i right in thinking that this is a problem with the recepient or could this still be an issue with our Exchange/DNS here?