SMTP XXXX problem

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SBS 2003 Std SP2, Exchange SP2
Trend Micro WFBS Advanced 5.1 w/ Smart Protection Network

Recently, I've had some internal users tell me they aren't receiving email from their customers/suppliers. Looking at the exchange logs, I see that the emails are being rejected due to 500 - UNRECOGNIZED COMMAND where it seems the remote mail server is sending XXXX instead of EHLO or HELO.
For example:

2009-06-03 13:35:35 65.118.56.42 - xxxx - +sv06tulbfpedge.bfpna.com 500 0 0 29 30016 SMTP - - - -
2009-06-03 13:35:35 65.118.56.42 - QUIT - - 240 30125 32 29 30016 SMTP - - - -

Looking through the logs, I actually see a lot of this happening and was wondering if this was the mechanism that the Trend Micro Smart Protection Network uses to reject connections to hosts that it deems are a source of spam. Of course, I've added the specific host IP addresses to every whitelist/acceptlist that I could find (Exchange Connection Filter, Smart Protection Network portal Approved List, WFBS Email Reputation Approved IP List) but it doesn't seem to make a difference... they are still rejected due to the XXXX command.

I find that some remote mail servers will initially issue the XXXX command followed up by a HELO command which processes the email as exepected. Some will not...

I'm certainly not an expert in this area but I'm hoping I can find someone that is...

-Scott

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Relevant Pages

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