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One of my clients is using a cPanel-based Virtual Dedicated Server that appears to allow unauthenticated SMTP:

bash$ echo EHLO | nc mail.clientscompany.com 25
250-SIZE 52428800
250 HELP

It therefore appears that anyone (esp. spammers) can use his mail server to send whatever - I just connected from my DSL connection at home, and...

bash$ nc mail.clientscompany.com 25
HELO clientscompany.com
MAIL FROM: realuser@clientscompany.com
RCPT TO: innocentspamvictim@wherever.com
From: <realuser@clientscompany.com>
To: <innocentspamvictim@wherever.com>
Date: ...
Subject: ...

I just tested this, and sure enough, it sent a mail from "realuser@clientscompany.com".

Since I am not familiar with cPanel and WHM, can someone provide pointers to configure his mail server to (a) only accept TLS connections and (b) only authenticated ones (i.e. with user/password, not just plain connections).

Thanks for any help.

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closed as off-topic by Hyppy, masegaloeh, mdpc, HBruijn, MadHatter May 4 at 7:57

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3 Answers 3


Without knowing what MTA your client is running its hard to provide more specific configuration information.

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I posted some more details in an answer below. –  ttsiodras Mar 3 '10 at 18:08
If you've authenticated to that server recently, it may not actually be an open relay. See webhostingtalk.com/showthread.php?t=671553 –  quadruplebucky Mar 4 '10 at 1:47

The VDS uses Exim:

bash$ yum list | grep -i exim
exim.i386                                4.69-23.1_cpanel_maild installed   

I just thought that cPanel/WHM would offer this though some menu/button which I am somehow missing... I have no trouble editing configuration files, but I fear that the set of pages (PHP/CGI scripts) offered by cPanel/WHM are depending on specific formating of configuration files (for Exim and the rest of the UNIX tools) - which I wouldn't want to break...

Any further advise most welcome.

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up vote 1 down vote accepted

Thank you "quadruplebucky", you were spot-on.

That is, false alarm - to anyone else who is worrying about this, just read link the link "quadruplebucky" pointed to.

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