How can I allow only one IP address to connect to my GMS server via SMTP?

Question

There could be a number of reasons as to why you would want to configure your mail system to receive connections from just one IP address. The most obvious reasons range from aiming to reduce the amount of spam from entering your network to your ISP delivering mail to your server.

Answer

Below you will find plenty of options available to facilitate this need.

1. If your ISP is able to offer you flexible port forwarding, you could configure the GMS SMTP service to listen on port 26 instead of the "well known" port 25. The
   ISP would then need to connect to your server via port 26 to deliver e-mail to you.

2. GMS Anti-Spam has a feature named "Allowed IPs". This is typically used to ALLOW incoming connections and to then block specific IP addresses from connecting. Conversely, it can be used to BLOCK connections but to allow "trusted" IP’s to connect. If you were to enter 1.2.3.4 into the listbox this would mean that only this IP can initiate an SMTP connection with your server. Be sure to read the context sensitive help available for "Allowed IP’s" to gain a better understanding of how to block specific IP addresses with use of the (!) exclamation character. The rejection will take place after the SMTP RCPT TO command.
3. If you wish to except connections that come from trusted IP’s and/or authenticated clients, this too is similarly achievable. This functionality is explained HERE.
4. You can use the IP Flexibility options built into GMS and configured under Administration > Mail > Incoming > Connections. This facility allows you to specify the IP addresses that are allowed to connect to each of the GMS services, the default is to allow any IP address but can easily be changed to allow particular IP addresses.
5. Finally, you could look into blocking IP’s at the Firewall so the connection should never even reach GMS.

See Also:

• Why are users not allowed to send mail via my server?
• What do the fields in ipconnect.txt mean?
• Why do I get an error relating to a file IPAllowed.txt after upgrading to build 3213?
• How do I use the Banned Hosts option?

Keywords:block banned smtp ban connections one single