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There is a mail server that I'm trying to test using telnet. I telnet to the server on the port and it responses with "220 **************************************************************************" The server is exchange and sending of mail requires authentication.

When I enter the command "helo" I get "500 5.3.3 Unrecognized command" no matter what I enter, even "quit" I get the same response. The website MXToolbox (http://mxtoolbox.com/diagnostic.aspx) says that the mail server is responding properly.

What could be wrong?

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2 Answers 2

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If the response truly is 

220 ******************************************
then the answer is that, contrary to what you might think, you are not talking to a Microsoft Exchange SMTP server.

Rather, you're talking to a Cisco router with the "mailguard" mechanism enabled, as Microsoft explains to those who think that they have Exchange problems. As Cisco says, if you want to talk to a Microsoft Exchange server — or to any other server that requires SMTP extensions such as SMTP AUTH, for that matter — you must disable the "mailguard" mechanism and let the world talk to your actual server.

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  • So then, how is mxtoolbox.com able to communicate with it properly?
    – user61925
    Jun 14, 2011 at 16:53
  • It's not exclusive to Cisco, although Cisco does respond with that. I think WatchGuard and/or SonicWall also utilize similar responses along with other SMTP proxies.
    – user48838
    Jun 14, 2011 at 18:01
  • Untrue. WatchGuard responds 220 SMTP service ready and SonicWall responds 220 smtp.example.com SMTP SonicWALL (w.x.y.z). Of the three, it is only Cisco routers that respond with the asterisks.
    – JdeBP
    Jun 15, 2011 at 15:23
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    mxtoolbox doesn't perform a full SMTP transaction, and isn't the acid test that you think it to be.
    – JdeBP
    Jun 15, 2011 at 15:26
  • What if I can connect fine from a Linux telnet session, but am getting the "500 unrecognized command" only from a Windows telnet session? Is that still explained by the mailguard issue?
    – TrojanName
    Mar 14, 2017 at 9:57
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You are more than likely running into a SMTP proxy, either on the router or firewall.

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  • "So then, how is mxtoolbox.com able to communicate with it properly?" It may be set to ESMTP-only in which case it would be "ehlo" instead of the "helo" command string.
    – user48838
    Jun 14, 2011 at 18:07

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