Building A PHP-Based Mail Client (part 2) - A Picture Is Worth A Thousand Words (
Page 2 of 7 )
The best way to
understand how message attachments work is with an example. Consider the
following email message, which contains no attachments,
Return-Path: <jane@some.domain.com>
Received: from localdomain (unknown [205.89.123.112])
by mail.domain.com (Postfix) with ESMTP id B0AYBHA41
for <john@some.domain.com>; Thu, 6 Dec 2001 21:05:51 +0530 (IST)
Message-Id: <3.3.7.32.20011206210943.00b26d40@melonfire.com>
Date: Thu, 06 Dec 2001 21:09:43 +0500
To: John Doe <john@some.domain.com>
From: Jane Doe <jane@some.domain.com>
Subject: Photos
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Do you still have those vacation photos with you? Send me a copy if you do.
- Jane
and then contrast it with this one, which does.
Return-Path: <john@some.domain.com>
Received: from localdomain (unknown [205.89.123.112])
by mail.domain.com (Postfix) with ESMTP id B0AYBHA41
for <jane@some.domain.com>; Thu, 6 Dec 2001 21:05:51 +0530 (IST)
Message-Id: <3.0.3.32.20011207082225.006e927c@localhost>
Date: Fri, 07 Dec 2001 08:22:25 +0500
To: Jane Doe <jane@some.domain.com>
From: John Doe <john@some.domain.com>
Subject: Re: Photos
Mime-Version: 1.0
Content-Type: multipart/mixed;
boundary="=====================_1007675545==_"
--=====================_1007675545==_
Content-Type: text/plain; charset="us-ascii"
Hi Jane,
Find photos attached.
John
--=====================_1007675545==_
Content-Type: application/zip; name="photos1.zip";
x-mac-type="705A4950"; x-mac-creator="705A4950"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="photos1.zip"
UEsDBBQAAAAIABythCvu1mnpugIAALsIAAALAAAAY29tcG9zZS5waHC1lltP2zAUx59Xqd/
-- snip --
AK6LAAAAAA==
--=====================_1007675545==_
Content-Type: application/zip; name="photos2.zip";
x-mac-type="705A4950"; x-mac-creator="705A4950"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="photos2.zip"
UEsDBBQAAAAIAACRDSux3m3PFwIAABsIAAAXAAAAc29hcGxpYi5zb2FwaW50ZXJvcC5waHC
-- snip --
cFBLBQYAAAAACwALALcCAABRfQAAAAA=
--=====================_1007675545==_--
As you can see, the message containing attachments differs
from the plaintext message in a couple of important ways.
1. Since it
consists of different parts (one for the body and one for each attachment), it's
Content-Type: header specifies the message type as "multipart/mixed".
Content-Type: multipart/mixed;
2. The various message sections are demarcated by a boundary
"marker"; this marker is unique to each message, and is defined in the main
message header so that MIME-compliant mail clients can distinguish between the
various message parts.
Content-Type: multipart/mixed;
boundary="=====================_1007675545==_"
If you look at the example above, you'll see that, as
specified in the message header, the boundary marker
"=====================_1007675545==_" is used as a separator between the
different parts of the message.
3. Since the SMTP protocol cannot handle
binary data, binary attachments - in this case, zip archives - must be encoded
into a text representation using one of a number of different encoding methods.
This information, together with information on the attachment type and original
filename, appears in the header for each message section.
Content-Type: application/zip; name="photos2.zip";
x-mac-type="705A4950"; x-mac-creator="705A4950"
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="photos2.zip"
This provides the mail client with all the information it
needs to decode and display the attachment. It can use the
Content-Transfer-Encoding: header to determine how best to decode the text block
back into binary data, the filename to determine what the resulting file should
be called, and the Content-Disposition: header to decide whether the binary data
should be displayed as an attachment or within the message body.
Note
that this discussion is restricted to MIME attachments only. Other standards to
handle mail attachments do exist. Take a look at
http://www.faqs.org/rfcs/rfc2045.html
for the MIME specification.
