Error when telnet used for testing

[mowen]

I am using Putty to try to access Zimbra using telnet to port 25. After the initial
220 xxxxx ESMTP Postfix

typing in
HELO test.com
produces an error:
502 5.5.2 Error: command not recognized

Repeating the HELO command straight away is successful and I can then proceed to send an email. Any ideas why there is an initial error or suggestions where to look to get an answer?

Thanks

[uxbod]

Does the same happen if you perform the telnet directly on the server ? When you perform the telnet are you going through a firewall at all ? When you get the initial connection if you hit return and then helo xyz.com does it work ?

[mowen]

Performing a telnet on the server does not give the error.
Yes, we go through an old 3com Superstack III to get to the DMZ where the mail server sits.
Hitting a return on connection produces the error, then the helo test.com works fine.

[uxbod]

Personally I feel that it is your switch which is munging the initial connection. This behavior has been seen elsewhere Postfix outside network 220 response and EHLO - LinuxQuestions.org.

[mowen]

Just found that when using putty to access Zimbra using Raw rather than Telnet on port 25, the error doesn't occur!

[uxbod]

Still reckon its your switch