SMTP – Getting ‘500 Unrecognized Command’ When Telnet to Mail Server

emailemail-serverexchangesmtpwindows

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?

Best Answer

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/

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