telnet mail.domain.ext 25<Enter>
Connected to mail.domain.ext.
Escape character is '^]'.
220 mail.domain.ext ESMTP Sendmail ?version-number?; ?date time gmtoffset?
HELO local.domain.name<Enter>
250 mail.domain.ext Hello local.domain.name [loc.al.i.p], pleased to meet you
MAIL FROM: mail@domain.ext<Enter>
250 2.1.0 mail@domain.ext... Sender ok
RCPT TO: mail@otherdomain.ext<Enter>
250 2.1.0 mail@otherdomain.ext... Recipient ok
DATA<Enter>
Subject: <subject of email><Enter>
From: <sender's email address><Enter>
To: <recipient's email addresses><Enter>
<Enter>
Body of email
<last blank line>
.<Enter> (single period on a newline followed by Enter)
QUIT<enter>
Another example
===============
telnet: > telnet mx1.example.com smtp
telnet: Trying 192.0.2.2...
telnet: Connected to mx1.example.com.
telnet: Escape character is '^]'.
server: 220 mx1.example.com ESMTP server ready Tue, 20 Jan 2004 22:33:36 0200
client: HELO client.example.com
server: 250 mx1.example.com
client: MAIL from: <sender@example.com>
server: 250 Sender <sender@example.com> Ok
client: RCPT to: <recipient@example.com>
server: 250 Recipient <recipient@example.com> Ok
client: DATA
server: 354 Ok Send data ending with <CRLF>.<CRLF>
client: From: sender@example.com
client: To: recipient@example.com
client: Subject: Test message
client:
client: This is a test message.
client: .
server: 250 Message received: 20040120203404.CCCC18555.mx1.example.com@client.example.com
client: QUIT
server: 221 mx1.example.com ESMTP server closing connection