Our office has been using MS Exchange Server for over 18 months now and I only just now discovered that our outgoing mail has never been going through our spam/antivirus service provider – d’oh! Fortunately, this turned out to be an easy fix – all I had to do was to use MMC to go to:
Organization Configuration ->
Hub Transport ->
Send Connectors ->
<your connector entry> ->
In our case, I had “Use domain name (DNS)…” selected, when I should’ve specified a smart host, using the FQDN provided to us by our filtering provider (note that a SMTP service restart is required for this change to take effect).
Thanks to this link for pointing me in the right direction on this one…