We can send mail via telnet using the following steps.
1. Telnet to hostname on port 25.
220 (then identifies itself – possibly with several lines of 220 + text)
2. HELO your_domain_name or whatever
250 (followed by human readable message)
3. MAIL FROM:firstname.lastname@example.org (ie, your email address)
250 is syntactically correct (or similar)
4. RCPT TO:them@someplace_else.com (email address you want to send to)
250 is syntactically correct
Tells you to send data then CRLF period CRLF at end
6. You type your message then CRLF period CRLF (ie, type a period on a line by itself then hit ENTER)
Mail is sent. That’s it.