When an email is sent to a server behind the watchguard, from another site that is protected with another watchguard the mail server returns. said: 500
Syntax error, command unrecognized. In this article it says that it may be a smtp-proxy problem https://www.experts-exchange.com/questions/20741604/There-was-a-SMTP-communication-problem-with-the-recipient's-email-server.html
what is the problem?