I'm currently building an app to intercept SPAM mail via an SMTP relay.  The only problem I've got is that some mail servers send a XXXX command prior to sending their HELO.  I've looked high and low to find out what the reason for the XXXX is, and haven't found anything anywhere.  My mail server just generates an error when it gets it.  Here's an example of the handshake just prior to an E-Mail being sent:

>  220 mail.(ourserver).com ESMTP Server (Microsoft Exchange Internet Mail Service 5.5.2655.55) ready[CR][LF]
>> XXXX grebe.mail.pas.earthlink.net[CR][LF]
> 500 Command not recognized.[CR][LF]
>> HELO grebe.mail.pas.earthlink.net[CR][LF]
>  250 OK[CR][LF]
>> MAIL FROM:<blah@earthlink.net>[CR][LF]
>  250 OK - mail from <blah@earthlink.net>[CR][LF]

That first XXXX command sent is what's losing me.  And it's not just earthlink that does it.  Like I said, I've searched high and low for something explaining it with no luck.  Best I'm able to guess is that for some reason maybe they WANT to generate an error response on the server for some reason.  Anyone with the answer gets 500 points.  (And no theories, please.  I need to know WHY they do this.)

-Javin