SMTP to Facebook

Question

Lately I've been playing around with telnet and found it quite interesting. Most interestingly, Facebook also uses SMTP. The IP is 66.220.155.11.

Some things about it are strange.

1. It doesn't take the HELO command.
2. You can only set the MAIL FROM: to a mail totally unlinked from Facebook.
3. It does take the DATA command, but I don't get which should be the headers.

In the end, even without the headers, the message is rejected. Any ideas on how to complete the relay?

Answer 1

Any business that wants to accept incoming email over the internet will need an SMTP server, so that facebook has one isn't unexpected. It is an incoming server, so won't accept facebook source addresses.

It appears to be fairly standard, and wants standard headers:

# telnet 66.220.155.11 25
Trying 66.220.155.11...
Connected to 66.220.155.11.
Escape character is '^]'.
220 smtpin.mx.facebook.com ESMTP
EHLO domain.com
250-smtpin.mx.facebook.com says EHLO to xx.xx.xx.xx:57628
250-8BITMIME
250-ENHANCEDSTATUSCODES
250 PIPELINING
MAIL FROM: paul@domain.com
RCPT TO: user@facebook.com
250 MAIL FROM accepted
250 RCPT TO accepted
DATA
354 continue.  finished with "\r\n.\r\n"
Message-ID: <499D11DC.1000706@airbred.com>
Date: Thu, 16 Jan 2013 19:01:32 +1100
From: Paul <paul@domain.com>
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-AU; rv:1.8.1.19) Gecko/20090122 Lightning/0.9 Thunderbird/2.0.0.19 
MIME-Version: 1.0
To: Paul <user@facebook.com>
Subject: Subject Line

Hello

.
250 OK A8/71-30754-8D667F05

This email came through to my facebook account without issue. Note that everyone on facebook has a facebook.com email address, and SMTP is how email is delivered on the internet, so the requirements for incoming mail on the facebook SMTP servers is much the same as any other - if they weren't, it wouldn't work.