An open mail relay is an SMTP server configured in such a way that it allows anyone on the Internet to send e-mail through it, not just mail destined to or originating from known users.[1][2][3] This used to be the default configuration in many mail servers; indeed, it was the way the Internet was initially set up, but open mail relays have become unpopular due to their exploitation by spammers and worms. Many relays were closed, or were placed on blacklists by other servers.
How to easily check if your mail server is open relay:
telnet relay-test.mail-abuse.org
[terung@myvps ~]$ telnet relay-test.mail-abuse.org
Trying 168.61.4.13...
Connected to relay-test.mail-abuse.org.
Escape character is '^]'.
Connecting to x.x.x.x ...
<<< 220 myvps.mydomain.com ESMTP
>>> HELO cygnus.mail-abuse.org
<<< 250 myvps.mydomain.com
:Relay test: #Quote test
>>> mail from:
<<< 250 ok
>>> rcpt to: <"nobody@mail-abuse.org">
<<< 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 1
>>> mail from:
<<< 250 ok
>>> rcpt to:
<<< 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 2
>>> mail from:
<<< 250 ok
>>> rcpt to:
<<< 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #test 3
>>> mail from:
<<< 250 ok
>>> rcpt to:
<<< 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 4
>>> mail from:
<<< 250 ok
>>> rcpt to:
<<< 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 5
>>> mail from: <>
<<< 250 ok
>>> rcpt to:
<<< 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 6
>>> mail from:
<<< 250 ok
>>> rcpt to:
<<< 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 7
>>> mail from:
<<< 250 ok
>>> rcpt to:
<<< 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 8
>>> mail from:
<<< 250 ok
>>> rcpt to:
<<< 553 we don't relay (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 9
>>> mail from:
<<< 250 ok
>>> rcpt to:
<<< 553 we don't relay (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 10
>>> mail from:
<<< 250 ok
>>> rcpt to: <"nobody@mail-abuse.org">
<<< 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 11
>>> mail from:
<<< 250 ok
>>> rcpt to: <"nobody%mail-abuse.org">
<<< 553 we don't relay (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 12
>>> mail from:
<<< 250 ok
>>> rcpt to:
<<< 553 we don't relay (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 13
>>> mail from:
<<< 250 ok
>>> rcpt to: <"nobody@mail-abuse.org"@[x.x.x.x]>
<<< 553 we don't relay (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 14
>>> mail from:
<<< 250 ok
>>> rcpt to:
<<< 553 we don't relay (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 15
>>> mail from:
<<< 250 ok
>>> rcpt to: <@:nobody@mail-abuse.org>
<<< 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 16
>>> mail from:
<<< 250 ok
>>> rcpt to: <@[x.x.x.x]:nobody@mail-abuse.org>
<<< 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #Test 17
>>> mail from:
<<< 250 ok
>>> rcpt to:
<<< 553 we don't relay (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #test 18
>>> mail from:
<<< 250 ok
>>> rcpt to:
<<< 553 we don't relay (#5.7.1)
>>> rset
<<< 250 flushed
:Relay test: #test 19
>>> mail from:
<<< 250 ok
>>> rcpt to:
<<< 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)
>>> rset
<<< 250 flushed
>>> QUIT
<<< 221 myvps.mydomain.com
Tested host banner: 220 myvps.mydomain.com ESMTP
System appeared to reject relay attempts
Connection closed by foreign host.
[terung@myvps ~]$
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