What is the basic SMTP protocol for sending mail?

RFC 2821 (http://www.faqs.org/rfcs/rfc2821.html) defines the SMTP protocol. To send a message you would normally open up port 25 on your mail server and send the following information:

HELO sending host
MAIL FROM: sender email
RCPT TO: recipient email
DATA
... the email message...
... any number of lines ...
.
QUIT

Between the DATA line and the . (period) line is the message. The period is alone on the line following the end of the message.