SMTP Protocol Returned a Permanent Error 503 - Duplicate HELO


An external sender received the following error when trying to email a legitimate recipient in my domain:  "Error transferring to MAILSERVER.DOMAIN.COM; SMTP Protocol Returned a Permanent Error 503 Duplicate HELO".

Could this be somehow caused by having multiple Exchange servers in the same Exchange org, with MailMarshal as the relay server to the Internet?
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I guess MX is pointing to "MailMarshal" ... and is receiving and forwards emails to Exchange ...
If that's the case and NDR is generated by Exchange ... make sure of Communication between MailMarshal & Exchange

From MailMarshal ... do SMTP Communication Test
Telnet to Port 25 to Test SMTP Communication

and try to submit the message
ShepsysAuthor Commented:
Correct.  The MX record is pointing to MailMarshal and is forwarding mail to/receiving mail from the Exchange servers.

Telnet tests work fine. This is the only error I have seen like this.  All other email traffic seems to be routing fine.  Just this one particular sender is having a problem.
If that's the problem then I hope you have tried that telnet test ... with his Email Address on "mail from: ..." ... and if that has worked then you can wash your hands ... its not your exchange problem.

you can carry out that same test from your home machine on your MX record ... and see if that "MailMarshal" is rejecting that message or what


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ShepsysAuthor Commented:
I have verified that telnetting with the sender's info works fine from outside the domain so, as you suggested, it must not be my exchange problem.

Thank you very much for your input.  Will close with an A.
We had a similar problem while using MS SMTP to send 3-4 email into one of our own email servers from a remote location and got around this by putting a 1 second pause between emails. Apparently the spam filter server we use (external provider) mistakenly saw the multiple email connections as a single connection with multiple HELO attempts.

Since this only began occuring at the precise moment the spam filter was enabled (and is corrected by disabled the spam filter) I look at it as a server problem - the spam filter server. It looks like the mail server software is poorly written or set too sensitively (or could have even been designed that way). After all, who wants multiple connects/disconnects to your server every 1/4 second? - this would be similar to a ping attack. Regardless, the only control the user would have, that I see, would be to put pauses between the emails - if they were using a script.

Would anyone have a better insight on this?
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