Getting "500 unrecognized command" when telnet to mail server from Windows - works fine from Linux

Question

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.

Windows:

C:\Users\Administrator>telnet
Microsoft Telnet> OPEN 192.168.1.7 25
220 ****************************************************************************
****************
EHLO
500 unrecognized command

Linux:

[me@mybox log]$ telnet 192.168.1.7 25
Trying 192.168.1.7...
Connected to 192.168.1.7 (192.168.1.7).
Escape character is '^]'.
220 ********************************************************************************************
EHLO me
250-mail1.mycorp.com Hello me [192.168.0.61]
250-SIZE 52428800
250-8BITMIME
250-PIPELINING
250-AUTH PLAIN LOGIN
250 XXXA

Can anyone help diagnose this issue on Windows?

Solution 1

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/

Solution 2

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.

Comments

[TrojanName]

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!

[Sembee]

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]

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.

[Peter VARGA]

Did you see this post - what can it be? Especially, the fix for PuTTY is interesting.

[Peter VARGA]

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.