ISA Firewalls and MDaemon — Outbound SMTP Problems


Dear Dr. Shinder,

First, I  would like be sorry for inconvenient to you. But I hope you could share your great experience to me.

My name, Bong Benly (Mr.). Benly is my first name. I live and work in Cambodia. At my institution, we use MDaemon (9.0.6) as mail server. Since I installed ISA server 2004, we could not send email to outside, and could not receive email from outside too. All email to outsiders, were get stuck in MDaemon.

I checked for solution from MDaemon website, the answer is as below:

There is an application filter for SMTP in ISA that needs to be disabled. Visit for more configuration information.

I still have no idea to get it work. I hope to hear from you. Thank you for your time.

With my best regards,




It seems strange that that the recommendation would be to disable the SMTP filter, since the SMTP filter provide vital security for published SMTP server. Keep in mind that the SMTP filter work only for Server Publishing Rules, not for outbound SMTP messages. So, for the techs at the MDaemon Web site to tell you to disable the SMTP filter to fix an outbound SMTP problem sounds ridiculous and indicates that they don’t understand how the ISA Firewall works, and they’re just guessing as to the nature of the problem.

The most common reasons for outbound mail to get “stuck” at the SMTP MTA include:

  • DNS problems. The SMTP server can’t resolve the name of the destination SMTP servers
  • Access Rule problems. You haven’t created a DNS server rule that allows outbound DNS, or you haven’t created a rule that allows outbound SMTP for the SMTP server
  • ISP problems. The ISP doesn’t allow you to send outbound SMTP. They might require that you use their SMTP relay

Bottom line: The probability that a correctly configured ISA Firewall is blocking outbound SMTP messages from going through is infinitesimally small.



Thomas W Shinder, M.D.


Email: [email protected]

MVP — Microsoft Firewalls (ISA)

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