A sample SMTP conversation.  You’ll need Putty (telnet mode!) in Windows, or Cygwin with ‘inetutils’ installed (for telnet), or the telnet for your Linux. The idea is that you are playing the role of a machine trying to send email through a mail server you select. You can learn causes of failure this way that you cannot learn any other way, or not at all easily.

• Blue is transmit.
• Red is receive. 
• We start with a telnet on port 25. The first step is a Linux command line as written, directly translatable to Cygwin’s telnet, and GUIable in Putty.
• The messages received will vary somewhat, but the initial numbers won’t. 
• The greater-than and less-than signs (< >) are required.
• Replace mail.domain.com with the FQDN or IP of the mail server. This is essential to get to the correct SMTP server.
• Replace test.domain.com with the Internet FQDN of the machine you are on if it exists, and otherwise, use test and the final domain name of the mail server you are trying to reach. This is how you are identifying yourself, as legitimately as available, to that mail server.
• Most SMTP servers, especially ESMTP servers, respond well to EHLO these days. A few still need the original, HELO.
• If it says OK at each step, the mail server has accepted what you have sent. Otherwise, it will give a short error, generally with a standard SMTP error code which you can look up.
• Choose the sender and the recipient carefully, according to what kind of test you are needing. For instance, if you are testing a simple mailbox, the FROM can be your own email address, and the TO can be a mailbox to which you have access, behind that mail server. Distribution lists and scan-to-email can indicate needed variations.

telnet mail.domain.com 25

220 mail.domain.com ESMTP

EHLO test.domain.com

250 test.domain.com

MAIL FROM: <sender@domain.com>

250 2.1.0 Sender OK

RCPT TO: <recipient@domain.com>

250 2.1.5 Ok

DATA

354 Enter mail, end with “.” on a line by itself

testing
testing
testing
.

250 2.0.0 Ok: queued as xxxxxxx