Messages delivered locally to the same domain on a Plesk for Windows Server behind NAT are being marked as SPAM by SpamAssassin

Symptoms

• Messages delivered locally to the same domain on a Windows Server behind NAT are being marked as SPAM by SpamAssassin.

• The following records can be found in mail headers:

  * 2.4 RDNS_NONE Delivered to internal network by a host with no rDNS
  * 2.5 TO_EQ_FM_DIRECT_MX To == From and direct-to-MX

Cause

This is expected behavior. Messages are being sent and received using internal network IP address, which has not published reverse DNS record, and the domain name for sender and recipient is the same.

Resolution

In order to disable these filters, perform the following:

1. Connect to the server via RDP.

2. Open the "%plesk_dir%AdditionalSpamAssassinetcmailspamassassinlocal.cf" file and add the following lines:

   score RDNS_NONE 0
   score TO_EQ_FM_DIRECT_MXR 0

3. Restart mail services using Plesk Services Monitor.

As an alternative solution, use “trusted_networks” directive.

A detailed description may be found in official documentation.

Examples:

trusted_networks 192.168/16 127/8 # all in 192.168.. and 127...*
trusted_networks 212.17.35.15 # just that host
trusted_networks 127. # all in 127...*