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.
How do I get around port 25? I don't receive my messages, since I need to use a proxy. Is that it? – OscarFR – 2013-01-17T03:51:28.900
1@OscarFR SMTP servers listen on port 25, so if you can't get a connection out on this port, you won't be able to telnet directly. – Paul – 2013-01-17T05:53:57.363