Since your SMTP module can accept incoming TCP connections, your server can be used by spammers as a mail relay engine: they can try to distribute their messages all over the world using your server, and they can also send a lot of unwanted messages to your users.
To protect your system from known spammer sites, CommuniGate Pro provides several methods to maintain "black lists" of offending hosts IP addresses.
When a "blacklisted" host connects to your server and tries to submit a message via SMTP, it gets an error message from your SMTP module and mail from that host is not accepted.
Note: connections from "blacklisted" hosts are still accepted. If you want to reject all connections from the certain Network Addresses, see the Denied Addresses section.
Use the WebAdmin Interface to open the Network pages in the Settings realm, then open the Blacklisted IP Addresses page.
Enter the IP Addresses of offending hosts in the Blacklisted IP Addresses field:
Each line can contain either one address:
Or an address range:
10.34.50.01 - 10.34.50.99
A comment can be placed at the end of a line, separated with the semicolon (;) symbol. A line starting with the semicolon symbol is a comment line, and is ignored.
It is difficult to keep the Server "blacklist" current. So-called RBL (Real-time Blackhole List) services can be used to check if an IP address is known as a source of spam.
Some ISPs have their own RBL servers running, but any RBL server known to have a decent blacklist can be used with your CommuniGate Pro server. Consult with your provider about the best RBL server available.
To use RBL servers, select the Use Blacklisting DNS Servers option and enter the exact domain name (not the IP address!) of the RBL server. Now, when the SMTP module accepts a connection from an IP address aa.bb.cc.dd, and this address is not listed in the Blacklisted, Unblacklistable, or Client Addresses lists, the module composes a fictitious domain name dd.cc.bb.aa.rbl-server-name where rbl-server-name is the domain name of the RBL server you have specified.
The SMTP module then tries to "resolve" this name into an IP address. If this operation succeeds and the retrieved IP address is in the 127.0.0.2-127.1.255.255 range, then the aa.bb.cc.dd address is considered to be blacklisted.
Note: this option results in an additional DNS (Domain Name System) operation and it can cause delays in incoming connection processing.