IP filtering for Mailserver

[moonworker]

I would like to use Promox Gateway as an IP address filter for incoming emails so that only specific internal devices can access the mail server directly. For this, I want to install a relay service. Since I don't know which external domain the device is sending maintenance requests to, I want to enter an asterisk (*) in the relay domain field, but this cannot be configured!

 

[MKle]

The standard configuration is

• port 26: outgoing email
• port 25: incoming mail

I have problems understanding your description, and assume:
You want to only allow some IP to send mails via the gateway → you want to limit access to port 26 to some IPs.

You could do that by adding a firewall configuration allowing only these IPs to access port 26.
If you do not want to use PMG for incoming mails, then you could simply block port 25