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

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.