Testing (spoofing) email addresses in Linux

SMTP dialog:

This is a sample of the dialog an e-mail client makes when connecting to an SMTP server for sending mail:

   telnet server-name 25     - SMTP communicates on port 25. See: /etc/services
   HELO your-domain          - This identifies the source of the mail.
   HELP                      - List the SMTP commands that are supported. (Included FYI and not part of a typical dialog)
   MAIL FROM: <your-email-address>
   RCPT TO: <recipient-email-address>
   DATA                      - End of DATA section is punctuated with a single dot on it's own line.
   Subject: E-mail-Subject
   Text of e-mail goes here
   .                         - The single dot
   QUIT