You have to stop relaying on mail server. That the job mail server has to do. I also run mail server on my network, behind the Mikrotik, and it works fine, without any special protection on router side. But I am in control of all domains hosted on that mail server, and it is easy for me to control outside connections with relaying intention.
However, if you run public server for hosting domains you do not control, then mail relaying is actually option that you cannot disable, as it is sure that some of your users want to use server form the outside network to send email to outside world.
You have some options to handle this too:
- one, check if your server may be set to allow SMTP connection which tries to relay, only if it was preceded with appropriate POP3 from the same IP (user has to authenticate on pop3). Option is usually called POP3 before SMTP.
- second, see if you may set mail server to require user to authenticate for SMTP connection too. That way, SMTP relaying would be allowed only to those who authenticated, meaning - your clients.
I agree with both of you. We will see what we can do in this matter, for now - use mentioned scripts.
Great news. I would like to see this option available in filter rules, and maybe even mangling. Some kind of caching results would be good too.