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@example.com, you are logged into your mail server mail.mydomain.com, and you are sending a message to firstname.lastname@example.org, it looks like this:
dig yourdomain.com mx and note the results (let's say it's mail001.yourdomain.com)
telnet mail001.yourdomain.com 25
- Deliver a message:
MAIL FROM: email@example.com
RCPT TO: firstname.lastname@example.org
RCPT TO: email@example.com
From: "Mehrdad" <firstname.lastname@example.org>
To: "Enrique Peñalosa" <email@example.com>
Cc: "Gabriela Peñalosa" <firstname.lastname@example.org>
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,
- 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.
- 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.