arvin1844

Member
Feb 4, 2012
8
0
51
cPanel Access Level
Root Administrator
example.com is a domain in my server. While testing using telnet, I can see example.com is open for relay and anyone can send mail from example.com without any authentication. How can we stop this?

Please see following test I have done.

Code:
#  telnet mail.example.com 25
Trying 10.10.10.10 ...
Connected to mail.example.com.
Escape character is '^]'.
220-mailserver.example.com ESMTP Exim 4.92 #2 Sat, 28 Sep 2019 06:46:11 -0400
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.
HELO any.name.com
250 mailserver.example.com Hello any.name.com [1.2.3.4]
MAIL From: <[email protected]>
250 OK
RCPT to: <[email protected]>
250 Accepted
DATA
354 Enter message, ending with "." on a line by itself
From: <[email protected]>
To: <[email protected]>
Subject: hello
.
250 OK id=1iEAFE-0002EJ-J2
QUIT
221 mailserver.example.com closing connection
Connection closed by foreign host.
 

Jcats

Well-Known Member
PartnerNOC
May 25, 2011
806
156
168
New Jersey
cPanel Access Level
DataCenter Provider
Did you do that locally within the server or you used telnet remotely? If local, try using a remote connection to relay.
 

Jcats

Well-Known Member
PartnerNOC
May 25, 2011
806
156
168
New Jersey
cPanel Access Level
DataCenter Provider
Can you PM me the server IP? cPanel by default does not act as an open relay.
 

quietFinn

Well-Known Member
Feb 4, 2006
1,299
127
193
Finland
cPanel Access Level
Root Administrator
example.com is a domain in my server. While testing using telnet, I can see example.com is open for relay and anyone can send mail from example.com without any authentication. How can we stop this?

Please see following test I have done.

Code:
#  telnet mail.example.com 25
Trying 10.10.10.10 ...
Connected to mail.example.com.
Escape character is '^]'.
220-mailserver.example.com ESMTP Exim 4.92 #2 Sat, 28 Sep 2019 06:46:11 -0400
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.
HELO any.name.com
250 mailserver.example.com Hello any.name.com [1.2.3.4]
MAIL From: <[email protected]>
250 OK
RCPT to: <[email protected]>
250 Accepted
DATA
354 Enter message, ending with "." on a line by itself
From: <[email protected]>
To: <[email protected]>
Subject: hello
.
250 OK id=1iEAFE-0002EJ-J2
QUIT
221 mailserver.example.com closing connection
Connection closed by foreign host.
As far as I understand you are not relaying email, you are connecting to mailserver.example.com and sending mail to [email protected], which is in the same server. You don't need to authenticate to do that.
 
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cPanelLauren

Product Owner
Staff member
Nov 14, 2017
13,296
1,271
313
Houston
An open relay is defined as the following:

An open relay (sometimes called an insecure relay or a third-party relay) is an SMTP e-mail server that allows third-party relay of e-mail messages. By processing mail that is neither for nor from a local user, an open relay makes it possible for an unscrupulous sender to route large volumes of spam.

So in order to be a relay your server needs to relay mail for domains that are not on the server. The proper test for this would be to do something like following:

Code:
$ telnet server.myserver.us 25
Trying <MYIPADDRESS>...
Connected to server.myserver.us.
Escape character is '^]'.
220-server.myserver.us ESMTP Exim 4.92 #2 Fri, 04 Oct 2019 11:37:51 -0500
220-We do not authorize the use of this system to transport unsolicited,
220 and/or bulk e-mail.
EHLO cpanel.net
250-server.myserver.us Hello cpanel.net [<LOCALIPADDRESS>]
250-SIZE 52428800
250-8BITMIME
250-PIPELINING
250-STARTTLS
250 HELP
MAIL FROM: [email protected]
250 OK
RCPT TO: [email protected]
550-Please turn on SMTP Authentication in your mail client.
550-hou-2.nat.myoffice.net (cpanel.net) [<LOCALIPADDRESS>]:35293 is not permitted
550 to relay through this server without authentication.
Where neither the MAIL FROM: or RCPT TO: domains exist on the server.

You can also use some tools that exist if you'd like to skip having to use telnet, mxtoolbox makes a really reliable Open Relay test which can be found here: Email Server Test - Online SMTP diagnostics tool - MxToolbox