Back in the old days we used to use this method to send mail all the time. That is beside the tendency to trick our less knowledgeable friends. They used to get mails from god@earth.net or devil@hell.org and so on.

What you basically need to make it work is an address of a mail server capable of relaying for you. That might be your local SMTP server or the one of your ISP. There are some open SMTP servers which will accept connections from anywhere but these are slowly disappearing.

Step 1 – Connect to the SMTP server
telnet mail.domain.com 25
The reply will be:
Connected to mail.domain.com
Escape character is ‘^]’.
220 mail.domain.ext ESMTP Sendmail …

Step 2 – Declare where you are sending the email from
HELO local.domain.name – Here is the nice trick, you can declare your local domain name as anything you want. Although you really should use your real fully qualified domain name as seen by the outside world, but the mail server has no choice but to take your word for it.
The reply should be
250 mail.domain.com Hello local.domain.name [loc.al.i.p], pleased to meet you
Step 3- Give your email address:
MAIL FROM: mail@domain.com
It should yield:
250 2.1.0 mail@domain.com… Sender ok
Step 4- Give the recipients address:
RCPT TO: mail@otherdomain.com
It should reply with:
250 2.1.0 mail@otherdomain.com… Recipient ok
Step 5 – To start composing the message issue the command:
DATA – In the new line after it you can type you actuall messege.
If you want a subject for your email type Subject: – Followed by a subject line and two enters.
Step 6 – To tell the mail server you completed the message enter a single “.” on a line on it’s own.
The mail server should reply with: 250 2.0.0 ???????? Message accepted for delivery
Step 7 – Close the connection by issuing the QUIT command.
The mailserver should reply with something like: 221 2.0.0 mail.domain.ext closing connection
Connection closed by foreign host.

Thats it. Try it and have fun.