Use socat to emulate an SMTP mail SERVER

socat TCP4-LISTEN:25,fork EXEC:'bash -c \"echo 220;sleep 1;echo 250;sleep 1;echo 250;sleep 1;echo 250;sleep 1;echo 354;sleep 1;echo 250; timeout 5 cat >> /tmp/socat.log\"'
Lots of scripts show you how to use socat to send an email to an SMTP server; this command actually emulates an SMTP server! It assumes the client is only sending to one recipient, and it's not at all smart, but it'll capture the email into a log file and the client will stop retrying. I used this to diagnose what emails were being sent by cron and subsequently discarded, but you can use it for all sorts of things.
Sample Output 
EHLO host.example.com
MAIL FROM:<root@example.com>
RCPT TO:<root@example.com>
DATA
Received: by host.example.com (Postfix, from userid 1234)
	id 955; Wed, 26 Nov 2014 12:56:03 -0800 (PST)
From: root@example.com (Cron Daemon)
To: root@example.com
Subject: Cron <root@example.com>    
Content-Type: text/plain; charset=UTF-8
Message-Id: <2014112.57348E5@host.example.com>
Date: Wed, 26 Nov 2014 12:56:01 -0800 (PST)

foobar: cannot remove user's mustache.
.
QUIT

1
By: pfudd
2014-11-26 21:14:05
cat echo sleep smtp socat Linux UNIX
0
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