> Most MTAs don't even allow it by default
qmail, postfix, Exchange 2003 and Exchange 2007 all accept email from the local domain to the local domain without authentication by default.
Some completely random tests. Small sites included as well. Surely not all these are "special" cases. Had much humour with mysql.com...
[root@lameserver ~]# dig mx +short cnn.com
10 atlmail3.turner.com.
[root@lameserver ~]# telnet atlmail3.turner.com 25
Trying 157.166.165.29...
Connected to atlmail3.turner.com (157.166.165.29).
Escape character is '^]'.
220 atlmail3 ESMTP
HELO testlol
250 atlmail3
MAIL FROM: imatcnn.com
250 sender <imatcnn.com> ok
RCPT TO: postmaster@cnn.com
250 recipient <postmaster@cnn.com> ok
500 #5.5.1 command not recognized
DATA
354 go ahead
here
.
250 ok: Message 438774241 accepted
QUIT
221 atlmail3
Connection closed by foreign host.
[root@lameserver ~]# dig +short mx mysql.com
20 mx3.sun.com.
20 mx4.sun.com.
5 btmx4.sun.com.
5 btmx6.sun.com.
[root@lameserver ~]# telnet mx3.sun.com 25
Trying 192.18.98.31...
telnet: connect to address 192.18.98.31: Connection refused
Trying 192.18.98.43...
telnet: connect to address 192.18.98.43: Connection refused
telnet: Unable to connect to remote host: Connection refused
[root@lameserver ~]# telnet mx4.sun.com 25
Trying 192.18.98.34...
[root@lameserver ~]# telnet btmx4.sun.com 25
Trying 192.5.209.6...
Connected to btmx4.sun.com (192.5.209.6).
Escape character is '^]'.
220 relay4i.sun.com (IntraStore TurboSendmail) ESMTP Service ready
HELO testlol
250 relay4i.sun.com G'day [207.192.72.19]! Why do you call yourself testlol?
MAIL FROM: someuser@mysql.com
250 sender <someuser@mysql.com> OK
RCPT TO: postmaster@mysql.com
250 recipient <postmaster@mysql.com> OK
DATA
354 Enter mail, end with "." on a line by itself
goes here
.
250 Message received and queued
QUIT
221 Until later [207.192.72.19]
Connection closed by foreign host.
[root@lameserver ~]# telnet mail.php.com 25
Trying 64.71.25.126...
Connected to mail.php.com (64.71.25.126).
Escape character is '^]'.
220 phpsparesv.php.com Microsoft ESMTP MAIL Service, Version: 6.0.3790.3959 ready at Thu, 11 Jun 2009 04:22:54 -0700
HELO lol.php.com
250 phpsparesv.php.com Hello [207.192.72.19]
MAIL FROM: loluser@php.com
250 2.1.0 loluser@php.com....Sender OK
RCPT TO: postmaster@php.com
250 2.1.5 postmaster@php.com
DATA
354 Start mail input; end with <CRLF>.<CRLF>
here
.
250 2.6.0 <PHPSPARESVBBsQwY6YH000003b2@phpsparesv.php.com> Queued mail for delivery
QUIT
221 2.0.0 phpsparesv.php.com Service closing transmission channel
Connection closed by foreign host.
[root@lameserver ~]# dig +short mx tpg.com.au
10 mx1.tpgi.com.au.
20 mx2.tpgi.com.au.
[root@lameserver ~]# telnet mx1.tpgi.com.au 25
Trying 203.12.160.122...
Connected to mx1.tpgi.com.au (203.12.160.122).
Escape character is '^]'.
220 mx1.tpgi.com.au ESMTP (mail9) Sendmail ready.
HELO imadomain
250 mx1.tpgi.com.au Hello loluser.pants.com [207.192.72.19], pleased to meet you
MAIL FROM: lolhere@tpg.com.au
553 5.1.0 lolhere@tpg.com.au... address does not conform to RFC 2821 syntax
MAIL FROM: <lolhere@tpg.com.au>
250 2.1.0 <lolhere@tpg.com.au>... Sender ok
RCPT TO: <postmaster@tpg.com.a>
550 5.7.1 <postmaster@tpg.com.a>... Relaying denied. Proper authentication required.
RCPT TO: <postmaster@tpg.com.au>
250 2.1.5 <postmaster@tpg.com.au>... Recipient ok
DATA
354 Enter mail, end with "." on a line by itself
here
.
250 2.0.0 n5BBOp8C024439 Message accepted for delivery
QUIT
221 2.0.0 mx1.tpgi.com.au closing connection
Connection closed by foreign host.
I could do this all day. I didn't pick and choose - these were random and I'd implore anyone to show me a single example of a server that's not "misconfigured". I thought you had me at one point - but it was my own typo on TPG. Although the blatantly forged mail going around did get my IP on several RBLs (it's not a mail server.. I can deal with that).