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

What Others Think

I found this extremely helpful, as it solves a problem I have with oldish hardware which cannot send email anymore due to modern restrictions (Auth,TLS,etc.), at the same time it made me aware of how primitive email transport really is. In my case the line as is didn't quite work, as it did not suffice to feed the client the response number, so the transfer was never terminated. The solution I am working with presently for an intelligent power-cord (NETIO-230B) follows. # socat TCP4-LISTEN:25,fork EXEC:'bash -c \"echo 220 welcome;sleep 1;echo 250 OK;sleep 1;echo 334 VXNlcm5hbWU6;sleep 1;echo 334 UGFzc3dvcmQ6;sleep 1;echo 235 2\.7\.0 Authentication successful;sleep 1;echo 250 OK;sleep 1;echo 250 OK;sleep 1;echo 354 Send;sleep 1;echo 250 OK;sleep 1;echo 221 BYE;timeout 5 cat >> /tmp/socat.log\"' The outlet has 4 watchdogs, which send email on failing devices. I can now either watch the file for alteration or extend the line to a small script that forwards the message to the real world email system. Some helpful advice is found here: "https://www.samlogic.net/articles/smtp-commands-reference.htm" and here "https://github.com/craSH/socat/blob/master/EXAMPLES" Thanks for the line, MulMic
MulMic · 11 weeks and 4 days ago

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Any thoughts on this command? Does it work on your machine? Can you do the same thing with only 14 characters?

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