by rob » Wed Mar 31, 2010 9:31 am
This is indeed possible. Basically users on your domain should ensure that when the send email via your SMTP, they use SMTP authentication. By default the SPAM settings are set to ignore users who do this. Then in the SPAM settings, you can enable the SPF options and ensure your domian has an SPF record. You can also enable the Sender Domain Check, which will also achieve the same affect. With a setup like this, only your real users can send mail from your domain to your server, all of which will bypass the SPAM settings, but any external sources will have SPAM filtering applied and also not be permitted to use your domain as the from.