I have a special setup and I can't really find information about this elsewhere. I have two ISPs. One ISP comes through on my standard IP address on my lan, 192.168.1.1. When pointing the MX record to the public facing IP that routes in through 192.168.1.1 I receive emails just fine. However, for my second ISP, I wanted to setup a 2nd backup MX record. I wanted to test what would happen if the first were to fail. When the first MX record fails and then it routes through the 2nd one, it routes through my other gateway on the LAN at 192.168.1.21.

It then gives me an error like this:

mail    | Apr  5 19:31:45 hostname postfix/smtpd[1493]: NOQUEUE: reject: RCPT from unknown[192.168.1.21]: 450 4.7.25 Client host rejected: cannot find your hostname, [192.168.1.21]; from=< [email protected]> to=<[email protected]> proto=ESMTP helo=<mail-ot1-f48.google.com>

How can I set it up to be able to accept emails routing through 192.168.1.21 as well as the original 192.168.1.1?