Troubleshooting Course Content Rough Drafts-How Does Email Work

Revision as of 00:00, 18 February 2015 by Haynesc (talk | contribs) (Simple Mail Transport Protocol)

What is e-mail?

Defining E-mail:

Often referred to as "e-mail", this term is frequently used or associated with the discussion of not only actual "mail" composition and retrieval, but is also used in the explanation of different protocols, standards, and architecture(s) associated with the electronic creation, submission, and retrieval of electronic messages.

We can see how "e-mail" is defined below;

messages distributed by electronic means from one computer user to one or more recipients via a   network.
send an e-mail to (someone).

In this training unit, we'll look at how e-mail came about, the industry standards and protocols used today as well as the architecture and vernacular of how e-mail works within your Zimbra deployment.

The origin of e-mail and industry standard protocols

History and Background:

Dating back to the early 1960's, the first form(s) of e-mail sprouted up in the form of services like AUTODIN, a legacy data communications service developed by the U.S. Department of Defense, and MIT's CTSS Mail, where remote terminals could dial-in to share and store files on a central disk(s). Informal methods of using these type(s) of systems later evolved to pass messages between terminal users.

The first "e-mail" systems used different features and ran on different systems that were not compatible with each other; coined "host-based e-mail systems". Most of these systems required users to be logged into in the same "terminal" or "mainframe" in order for communication to occur.

Subsequent to "host-based e-mail systems" came "LAN e-mail systems", those that ran on a local area network, which still required users to be logged into the same infrastructure. Some examples of "LAN e-mail systems" include Microsoft Mail and Lotus Notes. Further evolution allowed for these types of systems to communicate outside of an organization, assuming the same email system and proprietary protocols were in use.

Circa the 1970's, more of what we see in use today was in the origin of development. It was essential that as organizations grew larger and as technology advanced, those methods of mail exchange between remote sites or other organizations provided an avenue to transport messages of text globally via telecommunication links.

Internet Message Format:

Alas, through RFC, an "Internet Message Format" was derived and published by Qualcomm. RFC5322 is not the first RFC to establish an "IMF", albeit it's the most current.

From RFC5322:

  This document specifies the Internet Message Format (IMF), a syntax
  for text messages that are sent between computer users, within the
  framework of "electronic mail" messages.  This specification is a
  revision of Request For Comments (RFC) 2822, which itself superseded
  Request For Comments (RFC) 822, "Standard for the Format of ARPA
  Internet Text Messages", updating it to reflect current practice and
  incorporating incremental changes that were specified in other RFCs.

You can read more about this RFC here; RFC5322.

Beyond Text...Multimedia Attachments (MIME):

Further expansion led to "e-mail" that not only included text, but multimedia attachments as well. Established in RFC2045, through RFC2049, Multipurpose Internet Mail Extensions or "MIME" were derived.

Message Format:

Consisting of two major components, e-mail messages include both a "header" and a "body" section.

The Header:

This section is structured into fields, such as From, To, CC, Date, Subject, and other information pertaining to the message. An example "header" section is shown below:

Received: from (LHLO
( by with LMTP; Sat, 31 Jan 2015
01:05:53 -0800 (PST)
Received: from localhost (localhost [IPv6:::1])
by (Postfix) with ESMTP id 7970049F7
for <>; Sat, 31 Jan 2015 01:05:53 -0800 (PST)
X-Spam-Flag: NO
X-Spam-Score: -0.3
X-Spam-Status: No, score=-0.3 tagged_above=-10 required=6.6
tests=[ALL_TRUSTED=-1, BAYES_50=0.8, DKIM_SIGNED=0.1, DKIM_VALID=-0.1,
DKIM_VALID_AU=-0.1] autolearn=ham
Authentication-Results: (amavisd-new);
dkim=pass (1024-bit key)
Received: from ([IPv6:::1])
by localhost ( [IPv6:::1]) (amavisd-new, port 10032)
with ESMTP id rPQ4Stk2X1Sy for <>;
Sat, 31 Jan 2015 01:05:52 -0800 (PST)
Received: from localhost (localhost [IPv6:::1])
by (Postfix) with ESMTP id 3E2384A4C
for <>; Sat, 31 Jan 2015 01:05:52 -0800 (PST)
DKIM-Filter: OpenDKIM Filter v2.8.4 3E2384A4C
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed;; s=C3C5BF8A-F777-11E3-8DD3-AC39CE8301E4;
t=1422695152; bh=Md/ZQuH3wBFYNgW54KVDFg+v7hPPpgL/IhUsRmloLgo=;
X-Quarantine-ID: <24DhbFXYNwUn>
X-Amavis-Modified: Mail body modified (using disclaimer) -
X-Virus-Scanned: amavisd-new at
Received: from ([IPv6:::1])
by localhost ( [IPv6:::1]) (amavisd-new, port 10026)
with ESMTP id 24DhbFXYNwUn for <>;
Sat, 31 Jan 2015 01:05:51 -0800 (PST)
Received: from (localhost [])
by (Postfix) with ESMTP id E388C4A46
for <>; Sat, 31 Jan 2015 01:05:51 -0800 (PST)
Date: Sat, 31 Jan 2015 01:05:51 -0800 (PST)
From: admin <>
Message-ID: <>
In-Reply-To: <>
Subject: Re: ZCS Backup Report: SUCCESS
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Auto-Submitted: auto-replied (zimbra; vacation)
Precedence: bulk
X-Mailer: Zimbra 8.0.6_GA_5922
Thread-Topic: ZCS Backup Report: SUCCESS
Thread-Index: FP1J+yf4TQ4duzXiVDb92nMu0M2BmQ==

The Body:

Included in the body is basic content in form of unstructured text; possible containing a signature block at the end. The content of this section would be comparable to the "body" of a regular letter.

An example "body" section can be seen below:

During the Zimbra Admin Training class, we will be installing Zimbra on Amazon EC2 instances. 
Your Zimbra Training Administrator 
Zimbra | Community & Collaboration 
1855 S Grant Street, Suite 200, San Mateo, CA 94402 USA

The Message Header Explained

Every message has only one header structured into fields. Every field has a name and a value as outlined in RFC5322.

Header Fields:

A message header mustinclude the following fields:

•From: The email address, and optionally the name of the author(s). In many email clients not changeable except through changing account settings.

•Date: The local time and date when the message was written. Like the From: field, many email clients fill this in automatically when sending. The recipient's client may then display the time in the format and time zone local to him/her.

A message header should include the following fields:

•Message-ID: Also an automatically generated field; used to prevent multiple delivery and for reference in In-Reply-To: (see below).

•In-Reply-To: Message-ID of the message that this is a reply to. Used to link related messages together. This field only applies for reply messages.

Please refer to RFC3864 for elaboration on message header fields.

Simple Mail Transport Protocol

SMTP Explained:

The Simple Mail Trasnport Protocol, more commonly known as "SMTP", is an internet standard for e-mail transmission, first established in RFC821 and later expanded to include "Extended SMTP" via RFC5321; the protocol most widely used today.

SMTP by default uses port 25 for mail submission. Secure SMTP, known as SMTPS, defaults to port 465 in legacy configurations and port 587 in more current configurations.

The default values in a Zimbra deployment are port 25 for SMTP and port 587 for SMTP over TLS (SMTPS).

SMTP: Server vs. Client


Zimbra architecture employs a server side application called Postfix, which is an open source Mail Transfer Agent (MTA), for sending and receiving messages. The implementation of Postfix included with a Zimbra deployment is customized and not interchangeable with "standard" or "vanilla" Postfix installation(s).

Zimbra translates Postfix configuration variables for both users and server values into LDAP attributes and local configuration variables (zmlocalconifg read from /opt/zimbra/conf/localconfig.xml). It's important to keep in mind that any changes made to Postfix, outside of the Zimbra local configuration or modification of LDAP variables will not remain in place post upgrade.

The following files are customized for Zimbra's Postfix installation:

• - Modified to include the LDAP tables. The configuration script in the Zimbra MTA pulls   data from the Zimbra LDAP and modifies the Postfix configuration files.
• - Modified to use Amavisd-New.


Despite the primary focus of SMTP protocols being related to server side protocols, Client applications can also use SMTP for the submission of mail to an MTA server, or relay. In contrast, Client applications use POP or IMAP for the retrieval of mail.

Verified Against: Zimbra Collaboration Suite 8.6 Date Created: 01/22/2015
Article ID: Date Modified: 2015-02-18

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