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SMTP Bar Joke and EHLO

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Some time ago, I came up with a bar joke involving SMTP. Since I need to explain it a couple of times, I thought I just write it down as a blog post for future reference.

The joke goes like this (as a tweet):

The key thing here is the EHLO part. To explain this, let me show you a typical chatting between an SMTP server (e.g. from your mail provider) and an SMTP client (e.g. your email application). If you want to follow along, there is a nice trick. Sign up at Mailtrap for a test account (you can authenticate using your Github credential) and you will have a test server to play with.

Start by connecting to the server using telnet:

telnet 2525
Connected to
Escape character is '^]'.
220 ESMTP ready

ehloAt this moment, you are supposed to greet the server (see RFC 821, Section 3.5 on Opening and Closing) using the HELO command:


If you carefully read the above RFC 821, it is obvious that SMTP commands are 4-letter words. Thus, MAIL is for initiating a transaction, NOOP is to do nothing, HELP for showing up some instructions, and so on.

As SMTP grows in functionality, an extension mechanism is established so that the client recognizes certain extra features of the server and perhaps would like to leverage them. Rather than inventing a completely different opening command, EHLO is introduced (see RFC 5321, Section 3.2 on Client Initialization). This new command let the client and server know about each other’s privileged status. For example, running EHLO on Mailtrap gives us:

250-SIZE 5242880

which basically lists some service extensions supported by Mailtrap’s SMTP server.

Practically all modern email clients prefer to use EHLO instead. It is quite widespread and hence, the bar joke and the EHLO style of greeting. That was fun, right?


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