SmtpClient c = new SmtpClient( server );
c.from( sender );
c.to( recipient );
PrintStream p = c .startMessage();
p.println( "" );
p.println( "Subject: Test of SMTP Server" );
p.println( "" );
p.println( "Hi" );
c.closeServer();

SMTP is a kludgy anachronism. It lives on simply because everyone uses it. One of its major failings is a lack of authentication protocol. It has no official logon mechanism to prevent spammers from using your mail server to dispense SPAM.

Mail servers use various ad hoc schemes to protect themselves from unauthorised use:

1. Looking at the FROM: and REPLY-TO: in the mail header. This is so easy to defeat, newbies do it by mistake.
2. A manually maintained list of valid IPs permitted to use the server. Nasty people can still spoof IPs.
3. You login with POP3. Your IP goes into a temporary list of valid SMTP users. This means you must always attempt to pick up your mail immediately before sending any new mail.
4. Kerberos login.
5. A login command at the start of the SMTP session.
6. More advanced mail protocols.

RFC 821 describes the SMTP send mail protocol. You would use this to implement SendMail at the socket level with HELO commands and numbered responses.

If you talk to a mailserver, you can send mail to anyone in that domain without identifying yourself, however, if you want the mailserver to relay mail for you outside its domain, you need some way of identifying yourself. Mailservers drop off relayed mail using that same SMTP protocol.