Email Process

Adapted from a posting to the Procmail mailing list by Alan Stebbens Fri, 09 Aug 1996 13:19:52 -0700

To help clarify E-mail headers, here's a diagram of the E-mail process and its several steps:

sender -> MUA -> MTA -> (routing) -> MTA -> MDA -> {filtering} -> MUA -> receiver
[1]       [2]    [3]                        [4]                   [5]    [6]

MUA

Mail User Agent -- The program run by the user to read, reply to, compose and dispose of electronic mail (E-mail) messages. The MUA provides the interface between the user and the Message Transport Agent (MTA). Outgoing mail is handed over to an MTA for delivery while the incoming messages are picked up from a Mail Delivery Agent (MDA). Example of MUAs include the original UNIX mail program, /bin/mail, the Berkely Mail program and it's System V equivalent mailx, software programs like mush, elm, mutt, mh, Pine, Pegasus, and commerical programs like Eudora.

MTA

Mail Transport Agent or Message Transfer Agent -- The program responsible for storing and forwarding or delivering E-mail messages. Upon receiving a message from an MUA or another MTA it stores it locally and temporarily for analysis of the recipients and either delivers it to a local addressee or forwards it to another MTA. In either case it may edit and/or add to the message headers. It is usually referred to as the "mail server program." An example of this is UNIX sendmail or others including mmdf, smail, ZMailer, Exim, and Microsoft Exchange Server.

MDA

Mail Delivery Agent -- the actual program responsible for delivering mail to a user. MDAs usually handle one specific type of mail delivery. On UNIX systems this is typically the program /bin/mail, which is responsible for placing E-mail into a user's local mailbox file. Other MDAs include: uux for delivering UUCP mail; Procmail for advanced filtering of mail before delivery or maildrop which reads a mail message from standard input, then delivers the message to your mailbox.

Headers typically are provided by a "template" from the MUA to the sender when composing E-mail, usually during step [1]: See RFC2822

From:               # who is the sender
To:                 # who gets this mail
Cc:                 # people to keep informed, but need not respond
Bcc:                # other people to keep informed, keep secret
Subject:            # what the mail is about
Reply-To:           # added by originator to direct replies
Priority:           # try to influence speed and delivery
Precedence:         # set ordering in queue
Resent-To:          # used for redirecting (forwarding) E-mail
Resent-Cc:          # ditto
X-[something]:      # personalized headers

When the sender is done composing, and says "send it" to their mailer, some additional headers may get inserted by the MUA at step [2]: See RFC2045 for MIME (Multipurpose Internet Mail Extensions).

Date:               # day month year or dd mm yy hh:mm:ss zzz
Resent-Date:        # if being redirected (forwarded)
From:               # actual submittor
Sender:             # if a From: is already present
X-Mailer:           # what MUA composed this message
Mime-Version:       # if using a MIME capable mail program
Content-Type:       # type and subtype of data in message
Content-Transfer-Encoding:       # describe data type in message

When the MTA receives the E-mail from the MUA at step [3], it may insert additional headers showing the origination of the E-mail:

From                # if local E-mail, automatic or by filter options
Date                # if not already present
Message-Id:         # unique ID for the E-mail; the first MTA creates this
Received:           # trace routing of mail
Return-Path:        # shows how to get back to the sender

As each MTA hands off the E-mail, additional headers may get added, all as part of the MTA to >MTA hand-off in step [3]:

Received:           # inserted by each MTA for each message relay

As the final MTA hands the E-mail to a MDA, in step [4], there are still some more header insertions which may occur:

Apparently-To:      # added if no To: header exists
From                # may get added if local E-mail

Some sites insert special rewrite rules and mail filtering to occur to support virtual domains, and these header changes will occur at step [5], just before the incoming mail is dropped. Generally though, no new headers are added, except possibly by procmail to avoid loops, sort e-mail or auto-respond to a sender's e-mail request:

X-Loop:             # inserted to avoid filtering loops

Finally, at step [6] when the reader views their E-mail, most MUAs will apply a filter to the stored mail causing selected headers to be omitted from the display. In a sense then, this filtering "removes" the headers from the user's view (although no headers are actually removed by the MUA).

Example from Eudora before full header display:

Reply-To: "Somebody" 
From: "Somebody" 
To: 
Subject: Test message
Date: Fri, 2 Mar 2001 12:04:31 -0800
Organization: SomeOrganiztion
X-Mailer: Microsoft Outlook Express 5.00.2919.6600
X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2919.6600

Test message!!

The headers typically omitted are those inserted by the MTAs, and those having to do with the transport process and less with the contents.

Example from Eudora after full header display:

Received: from hawk.prod.itd.earthlink.net (hawk.prod.itd.earthlink.net [207.217.120.22])
        by no2.superb.net (8.11.1/8.11.1) with ESMTP id f22K5xv12202
        for ; Fri, 2 Mar 2001 15:05:59 -0500 (EST)
Received: from pacer2 (hsa184.pool015.at101.earthlink.net [216.249.86.184])
        by hawk.prod.itd.earthlink.net (EL-8_9_3_3/8.9.3) with SMTP id MAA14914
        for ; Fri, 2 Mar 2001 12:05:58 -0800 (PST)
Message-ID: <001301c0a353$feb287e0$a64cfea9@pacer2>
Reply-To: "Somebody" 
From: "Somebody" 
To: 
Subject: Test message
Date: Fri, 2 Mar 2001 12:04:31 -0800
Organization: SomeOrganiztion
X-Priority: 3
X-MSMail-Priority: Normal
X-Mailer: Microsoft Outlook Express 5.00.2919.6600
X-MimeOLE: Produced By Microsoft MimeOLE V5.00.2919.6600
Status: RO

Test message!!

Additional Interesting Articles

Creating Reports in SSRS, SSIS, and SQL
SQL Difference Between IS NULL and =NULL
C# DataSet v DataReader
Transactions Inside of SQL Stored Procedures
Looping Through a SQL Result in C#
SQL Check Existence of Table or Temp Table

©2008 AndrewKimball.com