Weird letters are coming. How to fix the problem?

Question

Domain Name System

Hello!
I have a domain geforester.ru
Recently, a lot of the same type of letters from non-existent mailboxes have been sent to domain mail:

As if someone is spamming on behalf of my domain. How to find and close this hole?
Sendmail is disabled on the server.
Suspicious email source code: https://gist.github.com/geforester/6873a7e129f8075...

Answer 1

you can specify in the googlemail settings, the IP address from which sending emails is allowed