Looks at the SMTP protocol, how it works and how to send messages with subjects, headers and attachments manually through a telnet session and includes some code for implementing a mail-sender in Java. http://xhz.freeshell.org/texts/smtp.html
These pages are designed as a reference for the Simple Mail Transfer Protocol (SMTP). They cover today's SMTP implementations and the IETF SMTP specifications, including some SMTP extensions. http://cr.yp.to/smtp.html
SMTP tutorial covering the Simple Mail Transfer Protocol and including sending a message; SMTP commands; a breakdown of envelope, header and body; and relay agents. Includes some diagrams. http://www2.rad.com/networks/1998/smtp/smtp.htm