I've seen the answer here to a similar question, but my situation is different in that it works fine when I telnet to the mail server from a Linux box, but I get this "500 unrecognized command" error when I telnet from Windows.


Microsoft Telnet> OPEN 25
220 ****************************************************************************
500 unrecognized command


[me@mybox log]$ telnet 25
Connected to (
Escape character is '^]'.
220 ********************************************************************************************
250-mail1.mycorp.com Hello me []
250-SIZE 52428800
250 XXXA

Can anyone help diagnose this issue on Windows?

  • Can you do the telnet localhost on the server, or did you not administrate that? I have seen that this error is sometimes cause by firewall features and / or some local software installed on the PC.
    – BastianW
    Mar 14, 2017 at 14:18
  • No, I don't have access to the mail server, and the guy who does isn't around this week unfortunately. I noticed that trying e.g. smtp.gmail.com 25 has the same result - what could that mean?
    – TrojanName
    Mar 14, 2017 at 15:04
  • Apologies, my last comment is incorrect. When I try to connect to gmail, I get: Connecting To smtp.gmail.com...Could not open connection to the host, on port 25
    – TrojanName
    Mar 14, 2017 at 15:10

2 Answers 2


There is a Cisco PIX/ASA involved somewhere. That is a firewall blocking the SMTP traffic. Odd behaviour when its SMTP scanning features are turned on.

Doesn't matter that the behaviour is different - the simple fact is the feature is enabled and needs to be turned off.

It is so common an issue that there is even a KB article from Microsoft about it! https://support.microsoft.com/en-gb/help/320027/

  • What is the downside to turning off this feature? I need to be fully prepared to explain a change like this to my firewall guy!
    – TrojanName
    Mar 14, 2017 at 15:35
  • As far as I am concerned, there is no downside. Email will work correctly, without delivery problems. I have never understood why the feature was used and it gives so much hassle that turning it off is the best option.
    – Sembee
    Mar 14, 2017 at 21:05
  • The link is dead. Can you please update it? Thx Jul 10, 2019 at 18:13
  • The article has been pulled by Microsoft it would appear. However if you Google for fixup SMTP then you will find plenty of other referenced.
    – Sembee
    Jul 11, 2019 at 16:45
  • Did you see this post - what can it be? Especially, the fix for PuTTY is interesting. Jul 16, 2019 at 15:08

In my experience this error is often caused by a firewall feature which is blocking/scanning the SMTP traffic.

You should check that with your firewall administrator (if he can switch that off). You can also try to find a connection which isn´t using this firewall to double check that.

  • Did you see this post - what can it be? Especially, the fix for PuTTY is interesting. How can the Passive Mode disable any firewall settings so it works? I see here the difference but I don't understand why it should help. Jul 17, 2019 at 11:25

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