WinDeveloper IMF Tune

WinDeveloper IMF Tune
WinDeveloper IMF Tune

Telnet Port 25

Alexander Zammit

Alexander Zammit Photo

A Software Development Consultant with over 20 years of experience. Many of his projects involved Exchange integrated applications, including a FAX server, a mail security product and anti-spam products.

  • Published: Oct 30, 2006
  • Category: General
  • Votes: 1.5 out of 5 - 32 Votes
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Telnet allows submitting emails to any SMTP server manually. Stepping through the SMTP protocol is an excellent opportunity to test and troubleshoot an email server. Here is what any email administrator should know.

We are now ready to submit the email content. The server is alerted of this through the DATA command.

DATA

The server responds showing it is now waiting for the email content. All the text entered at the console will add up to it. This goes on until a special character sequence is entered:
<CRLF>.<CRLF>

<CRLF> stands for carriage return line feed. This is what you get when hitting the Enter key. Thus the end of data marker is simply a line containing only a period.

The content of a simple SMTP email is composed of a bunch of headers followed by the body text. This is covered in the RFC 2822 specifications. More complex content including attachments and an HTML body are typically submitted with the help of MIME encoding. The MIME specifications are covered by yet another set of RFCs. However this is again beyond our scope. So we just limit ourselves to the most popular email headers (From, To, Subject) and a simple text body:

From: Sender <sender@exchangeinbox.com>
To: <rcp@adminstop.com>, <rcp2@adminstop.com>
Subject: Telnet Port 25

This is a test email
bye
Alex
.

Headers start with a name immediately followed by a colon. Note the empty line following the subject header. This indicates the beginning of the body text. If missing, Exchange will normally still be able to identify the correct body start position. Also note the final period indicating the end of content data.

When performing a simple test, we don't really need to remember any syntax rules for the email body. We could have easily just entered:

This is a test email
.

Exchange will still be able to deliver this. Just be ready to seem some "less than perfect" emails at the recipient mailbox.

End Of Data

On detecting the end of data sequence the SMTP server queues the email for delivery. We could now submit another email starting straight from the MAIL command. When ready the QUIT command closes the connection.

This completes submitting an email in a scenario where everything is plain sailing. However, as we shall see in an upcoming article, things get more interesting when testing an email rejection or when verifying our Exchange configuration.

MAIL FROM, RCPT TO and Email Headers

From this discussion we have the opportunity to highlight an important point that tends to confuse some email administrators. In this SMTP session we specified the sender address twice, first at the MAIL command and next at the From content header. The same goes for recipient addresses. We had the same addresses on submitting RCPT TO and the To content header.

An SMTP server is only concerned with addressing information delivered through MAIL and RCPT protocol commands. It does not interpret any addressing information within content headers. Headers are what email clients see and display at their interface.

Indeed the email headers could contain anything. Legitimate email sources will typically provide consistent addressing data. However spammers often jumble these in all manners. What has to be valid are the recipient addresses specified by the RCPT protocol commands since these determine the destination mailboxes. This means that what email clients show may be completely unrelated to what an SMTP server operates on.

Final Tips

Today we looked at the basics of SMTP and used telnet to submit an email. We focused on the bare essentials that are most important in everyday use. For a thorough understanding of SMTP it is best to check RFC2821. Watch out for an upcoming article that will use telnet to test various Exchange 2003 features.

Reference

RFC 2821 - Simple Mail Transfer Protocol

RFC 2822 - Internet Message Format

User Comments - Page 1 of 1

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Alexander Zammit 1 Feb 2008 02:02
There is no magic command that will allow you to relay otherwise spammers would make a big party.

Relaying must be enabled at the exchange server see details from here:
http://www.exchangeinbox.com/article.aspx?i=70

VRFY is used to test if an email address exists on a server and RSET resets the SMTP session so that you could restart issuing the command sequence.

Check the RFC here for complete details:
http://www.faqs.org/rfcs/rfc2821.html

kamarul 31 Jan 2008 17:31
is there any commands to allow relaying in the telnet <ip address> 25 session? what is the function of the other commands such as VERFY, RSET, etc
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