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user541686
Asked: 2011-08-14 22:09:50 +0800 CST

How to use SMTP to send mail to some host?

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I'm trying to use SMTP to send mail to my account on some server (say, on Gmail).

So far I've tried:

220 mx.google.com ESMTP y14sm3079810ibf.45
HELO
250 mx.google.com at your service
STARTTLS
220 2.0.0 Ready to start TLS
AUTH LOGIN
Connection closed by foreign host.

What am I doing wrong, and how can I fix it so I send an email as though I were my own server?

smtp

3 Answers

  1. Best Answer

    Just submitting the STARTTLS command doesn't put the connection into TLS mode. You have to negotiate and implement it properly.

    • 3

  2. Gmail requires TLS to relay mail on your behalf. This type of connection is necessary when your desktop or mobile e-mail client is sending mail from your Gmail account.

    However, any normal mail server will still accept a message to a recipient on that server without using TLS or any kind of authentication. For example, you can connect to gmail-smtp-in.l.google.com on port 25 and conduct an unencrypted SMTP session.

    Assuming that you are [email protected], you are logged into your mail server mail.mydomain.com, and you are sending a message to [email protected], it looks like this:

    1. dig yourdomain.com mx and note the results (let's say it's mail001.yourdomain.com)
    2. telnet mail001.yourdomain.com 25
    3. Deliver a message:
    HELO mail.mydomain.com
MAIL FROM: [email protected]
RCPT TO: [email protected]
RCPT TO: [email protected]
DATA

From: "Mehrdad" <[email protected]>
To: "Enrique Peñalosa" <[email protected]>
Cc: "Gabriela Peñalosa" <[email protected]>
Date: Fri, 30 Sep 2011 11:21:19 -0700
Subject: Thanks for all the fish!

It was a wonderful picnic. I really enjoyed the salmon burgers.

See you next week,

Mehrdad

.

    Caveats:

    1. Backspace does not work in an SMTP session! You are supposed to be a server, and servers don't make typos. If you are experimenting by hand, it is much better to compose your inputs in advance so you can copy and paste each line from a text editor.
    2. Because your messages won't necessarily look like they are coming from a genuine mail server -- for example, the receiving server may check things like your reverse DNS and SPF records -- you may inadvertently run afoul of spam protection on the receiving end. In the case of a destination like Gmail, your message is almost guaranteed to be detected as a forgery and sent directly to the spam folder.

    To learn more, see the Wikipedia article.

    • 2

  3. You issued the HELO command to your SMTP server. This tells it that you want to use the original SMTP commands. STARTTLS is not one of these commands, thus you should not be able to use STARTTLS.

    The first step in resolving this is to switch from the HELO command to the EHLO command. When you do, you will see that the SMTP server responds with the set of SMTP extensions that it supports. Hopefully, STARTTLS will be listed.

    Here's a conversation with GMail's SMTP server:

    CLIENT: EHLO me.example.com
SERVER: 250-mx.google.com at your service
SERVER: 250-SIZE 35882577
SERVER: 250-8BITMIME
SERVER: 250-STARTTLS
SERVER: 250 ENHANCEDSTATUSCODES
CLIENT: STARTTLS
SERVER: 220 2.0.0 Ready to start TLS
<negotiation begins here...>
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