We currently have a few lines and therefore a few separate IP addresses within the MX groupings. So it was relatively easy for us to try this out. We just disabled Port 25 (smtp) on the main mail server. We initially just left it at that, without a response to the request to the sender. This meant that the delivery was delayed, as the sender had to wait one to two minutes before timing out to the "dummy" mailserver.
You can test this by telnetting to the port: 'telnet mail.host.com 25' and that would take forever before it returns any results.
Therefore, to be a better 'netizen, and as recommended by the No listing page, we have requested the server to return a "Reject" on any connections to port 25. This means that the connections are immediately terminated, and non-spam senders will immediately try MX record #2.
Additionally, instead of doing this blind, we have current statistics of spam rates collected over the year on some selected email accounts. So here is the graph of spam (red) vs ham (everything else) for the past two weeks:
As you can see spam for these accounts are at a "healthy" 80-90%. We shall see in a few weeks if the changes made to the MX records make a difference to our spam rates!
yk.
