Introduction
This is part 2 of my articles about email receiving with POP3 and MIME processing. My first article POP3 Email Client (.NET 2.0) covered the reliable downloading of emails from POP3 servers, which left us with a pure ASCII representation of the email body. This was the easier part.
In this article I provide the code to split the raw ASCII email into body, attachment, alternate views, etc. This was much harder to do, because while the POP3 specification is simple and specified straight forward in one RFC, there are several MIME related RFCs, which provide a multitude of possibilities how simple stuff like an email's actual text can be sent. The MIME specification allows for great flexibility, but Microsoft, being Microsoft, of course supports only a subset (for example no recursion of MIME parts within MIME parts). The provided code supports both worlds completely and gives the programmer the flexibility to access information about the received email as needed.
If you wonder why I wrote this article despite the fact that there are various articles on CodeProject for MIME support, here are some of the shortcomings encountered:
- some code is not managed
- use of DLLs without .NET source code
- too limited functionality
- no integration with
System.Net.Mail.MailMessage - no XML documentation, etc.
My code is based on the following work:
Background
Structure of a simple email
A simple email in pure ASCII might look like this:
Date: Sat, 2 Sep 2006 17:25:15 +0200
From: Sender@NoSpam.com
To: Receiver@NoSpam.com
Subject: simple plain text mail
Just a plain text email
.
The first 4 lines are called the header of the email and they are separated from the body by an empty line. The end of the email is marked with a line containing just one "." (a period sign). There will be many more header lines when you look at a real email, some RFC standard ones and others, like this one from GMail:
X-Gmail-Received: f105c784e77f8b689759558db72ccd07f60387ba
Introduction of MIME
In the beginning there were just plain ASCII emails as defined in
RFC 2822. Plain ASCII was soon not sufficient, though, and the Multipurpose Internet Mail Extensions specification MIME was created to support non US-ASCII texts, multi-part message bodies, rich text (HTML), images, sounds and attachments. The specification tried to offer great flexibility and to cater to all kind of possibilities. The result was numerous RFCs (2045, 2046, 2047, 2049, 2231, 2387, 4288, 4289, ... ). As it often happens in big groups, the whole thing became rather complicated and, even worse, left it to the implementer how precisely body text, etc. are implemented.
In order to help you with the extraction of information from MIME based emails, I'm going to explain to you the basic MIME principles. First let's have a look at a complete MIME email. It might be a bit confusing, but it gives a good overview of the various MIME elements which I will explain one by one. This email has one email header, followed by the email body text and a .GIF picture. Notice the "--0-494165446-1157210079=:74253" line, which separates the various parts of the email, called MIME entities.
Date: Sat, 2 Sep 2006 17:25:15 +0200
From: Sender@NoSpam.com
To: Receiver@NoSpam.com
Subject: simple gmail mail
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="0-494165446-1157210079=:74253"
Content-Transfer-Encoding: 8bit
--0-494165446-1157210079=:74253
Content-Type: text/plain; charset=iso-8859-1
Content-Transfer-Encoding: 8bit
Content-Disposition: inline
This is the email body
This email has a smallPic.gif attachment
--0-494165446-1157210079=:74253
Content-Type: image/gif; name="SmallPic.GIF"
Content-Transfer-Encoding: base64
Content-Description: 437081412-SmallPic.GIF
Content-Disposition: inline; filename="SmallPic.GIF"
R0lGODlhQQBBAPcAAAAAAIAAAACAAICAAAAAgIAAgACAgICAgMDAwP8AAAD/
NZWZfpnCck/OeTUXvUdXxdi9/SbDPFS4t+/fwIMLH068uPHjyJMrX868ufPn
0KNLn069uvWVAQEAOw==
--0-494165446-1157210079=:74253--
.
Structure of an email header field
An email header field as defined in RFC 2822 has the following structure:
field-name ":" [ field-body ] CRLF
Example:
MIME-Version: 1.0
"MIME-Version" is the field-name, "1.0" is the field-body. The MIME-Version header field is mandatory for every MIME email. All other MIME header fields start with "Content-..."
Content-Type
The most powerful MIME header field is the Content-Type and is defined in RFC 2046. It can look like this:
Content-Type: text/plain;
Content-Type: text/plain; charset=ISO-8859-1
Content-Type: text/plain; charset=us-ascii
Content-Type: text/plain; charset=utf-8
Content-Type: text/html;
Content-Type: text/html; charset=ISO-8859-1
Content-Type: text/css
Content-Type: image/gif; name=image004.gif
Content-Type: image/jpeg; name="image005.jpg"
Content-Type: message/delivery-status
Content-Type: message/rfc822
Content-Type: audio/x-mpeg
Content-Type: video/mpeg-2
Content-Type: application/msword
Content-Type: application/mspowerpoint
Content-Type: application/zip
Content-Type: multipart/mixed;
boundary="----=_Part_3431_12384933.1139387792352"
Content-Type: multipart/alternative;
boundary="----=_Part_4088_29304219.1115463798628"
Content-Type: multipart/related;
boundary="----=_Part_2067_9241611.1139322711488"
Content-Type: multipart/digest;
boundary="----=Next message 15543233913938263541"
Content-Type: multipart/report; report-type=delivery-status;
boundary="k04G6HJ9025016.1136391237/carbon.singnet.com.sg"
Content-Type: multipart/parallel
The Content-Type field is used to specify the nature of the data in the body of a MIME entity, by indicating media type and subtype identifiers, and by providing auxiliary information that may be required for certain media types. Some of the media types are:
- text
- image
- message
- audio
- application
- multipart
Each of the media type defines its own set of subtypes, which might be followed by a set of parameters, each specified in an attribute=value pair. For example:
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
The media type is "text", the subtype is "plain", the attribute is "charset" and the attribute value is "ISO-8859-1". There could be more attribute=value pairs like "format=flowed".
Content-Type Multipart
The media type "multipart" provides the flexibility to split an email into several parts, like plain text, HTML text and attached files. There are versions of multipart (subtypes), but all have the same attribute "boundary". Its value is a string which is unique in the whole email and is used for marking the boundary delimiter lines of the various parts. Let's look at the previous example again, this time only with Content-Type information lines:
Headerlines
Content-Type: multipart/mixed; boundary="0-494165446-1157210079=:74253"
Headerlines
--0-494165446-1157210079=:74253
Content-Type: text/plain; charset=iso-8859-1
Other MIME part header lines
The plain text email body
--0-494165446-1157210079=:74253
Content-Type: image/gif; name="SmallPic.GIF"
Other MIME part header lines
The attachment coded in Base64
--0-494165446-1157210079=:74253--
.
The first 3 lines are part of the email header. The end of the header is marked by an empty line. All other lines are part of the email body, which ends with the line having only a "." (period). The boundary delimiter line breaks the body itself into the email text and the file attachment. This line always starts with "--" followed by the boundary string. The last boundary delimiter line is followed by trailing "--".
Each MIME entity has a entity-header and a entity-body separated by an empty line. Since emails and MIME entities use the same structure and the same kind of header lines, it is possible that whole emails can become a MIME entity, which is useful for mail systems (Content-Type: message). But of course having an email in another email in another email leads to many complications, and so it is no wonder that most mail program use a different solution for forwarding an email, they just merge it with the email text body. This has the advantage that even mail clients not supporting MIME can handle forwarding. Similarly, even the MIME specification is recursive, Microsoft's System.Net.Mail.MailMessage is not ! More about this later.
Content-Type: multipart/mixed
Often the top most multipart subtype is "mixed". It indicates that the email consists of several MIME entities, without specifying anything more about the kind of entities. "multipart/mixed" is used as default, if the actual subtype is not recognised by the email client.
Content-Type: multipart/alternative
The subtype "alternative" is used, if the same email is sent in plain text and HTML. Both have the same content, but in alternative coding. The email client is supposed to display to the user the last alternative part understood by the client. If an email consist of a plain text entity followed by an HTML entity, the email client is supposed to display the HTML text, even if it also knows how to display plain text, because the HTML version came last. An email with plain text and HTML can look like this:
Some header lines
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="----_=_NextPart_001_01C6CEA2.EF9BECF8"
------_=_NextPart_001_01C6CEA2.EF9BECF8
Content-Type: multipart/alternative;
boundary="----_=_NextPart_002_01C6CEA2.EF9BECF8"
------_=_NextPart_002_01C6CEA2.EF9BECF8
Content-Type: text/plain; charset="iso-8859-1"
HTML sample email with bold text and attachment.
------_=_NextPart_002_01C6CEA2.EF9BECF8
Content-Type: text/html; charset="iso-8859-1"
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<STYLE>
DIV { FONT-SIZE: 10pt;
FONT-FAMILY: Verdana, Arial, Helvetica, sans-serif }
</STYLE>
</HEAD>
<BODY>
<DIV>
HTML sample email with <STRONG>bold</STRONG> text and attachment.
</DIV>
</BODY>
</HTML>
------_=_NextPart_002_01C6CEA2.EF9BECF8--
------_=_NextPart_001_01C6CEA2.EF9BECF8
Content-Type: image/gif; name="SmallPic.GIF"
R0lGODlhQQBBAPcAAAAAAIAAAACAAICAAAAAgIAAgACAgICAgMDAwP8AAAD/
NZWZfpnCck/OeTUXvUdXxdi9/SbDPFS4t+/fwIMLH068uPHjyJMrX868ufPn
0KNLn069uvWVAQEAOw==
------_=_NextPart_001_01C6CEA2.EF9BECF8--
.
The structure of this email is:
multipart/mixed
| multipart/alternative
| | text/plain; format=flowed; charset=ISO-8859-1
| | text/html; charset=ISO-8859-1
| image/gif; name=SmallPic.GIF
Notice that the picture is part of multipart/mixed, not multipart/alternative.
Content-Type: multipart/related
Multipart-related can be used to send HTML text and graphics or other related material in the same email. It is beyond the scope of this article to explain the details of any other of the multipart mediatypes.
Content-Transfer-Encoding
POP3 defines that the body of an email is 7bit US ASCII code. Since the text displayed to the user can be any Unicode and file attachments are usually array of bytes, the email sender must encode this content to ASCII and we, the receiver of the email, need to decode it. If the value is "7bit", no encoding was used. "8bit", or "binary" has the same meaning, but is not supported by the .NET framework. I treat "8bit" like "7bit", i.e. take the content as it is, whereas "binary" is illegal in POP3, because some character sequences like CRLF "." CRLF have a special meaning in POP3, but might occur in random binary.
Content-Transfer-Encoding: quoted-printable
If a MIME entity consists mostly of US ASCII characters, it is enough to encode just some special characters and all bytes not covered by the US ASCII characterset. "quoted-printable" does this by sending a "=" and the hex value of the byte as ASCII characters. A carriage return (hex: 0D) becomes: "=0D". There are a number of rules dealing with special circumstances. I couldn't find a decoder for quoted-printable in .NET, so I copied the QuotedPrintable Class, by Bill Gearhart source code from ASP emporium.
Content-Transfer-Encoding: base64
Base64 uses a limited set of characters ("A"-"Z", "a"-"z", "0"-"9", "+", "/") to express a 6 bit value. Any 3 bytes can be expressed with 4 encoding characters. As an example, let's take the first 4 ASCII characters "R0lG" of the graphic file in our example email:
R 0 l G
001001 110100 100101 000110
Resulting 3 bytes:
00100111 01001001 01000110
Details can be found at RFC 1421, 4.3.2.4 Step 4: Printable Encoding
Using the code
The best way to get an understanding of a library is to use it. The Main function in the downloadable code does just that. It connects to an POP3 server (don't forget to provide the proper server name, user name and password) and downloads at most 5 emails. The code will not delete the emails from the server, but the server might delete them anyway, depending on its settings. The structure of the 5 emails will be displayed on the console. "Program.cs" also contains the method SendTestmail() to generate some sample emails.
Emails are received by Pop3MimeClient derived from Pop3MailClient, which is described in POP3 Email Client (.NET 2.0), by Peter Huber and offers all the functionality to interact with the POP3 server. Pop3MimeClient adds the method GetEmail which fetches one particular email from the POP3 server and returns it decoded as RxMailMessage.
Mapping MIME to System.Net.Mail.MailMessage
The System.Net.Mail.MailMessage class is used by System.Net.Mail.SmtpClient for sending emails with SMTP. MailMessage includes only the information needed to send an email. Receiving an email creates some additional information. Therefore, a new class RxMailMessage is inherited deriving from MailMessage and adding properties like DeliveryDate or DeliveredTo.
The SmtpClient converts MailMessage to a MIME conformant email, but MailMessage provides hardly access to any MIME related functionality. When receiving an email, we would like to store the complete information. Pop3MimeClient receives the first MIME entity by MIME entity and stores them as a MIME entity tree in the new Entities collection property of RxMailMessage. If possible, the info is also copied to the properties inherited from MailMessage. This gives the user the freedom to choose if the complete email in MIME form is used for further processing or just the simpler, but possibly incomplete Body, AlternateViews or Attachments as defined by MailMessage. The method decodeEntity can be used as an example how to loop through all MIME entities of an email.
History
11.10.2006 Improvements Constructor, Handling ContentDisposition==null
- Proper handling of
useSSL in Pop3MimeClient constructor - Prevent exception when
ContentDisposition is null
8.10.2006 Improvements Attachment Handling
- Detecting content-disposition header field and creating an attachment if it looks like: "
C-Disp: attachment" - Only MIME entities from
multipart/alternative parents become alternative views - Added end markers for multiparts in
RxMailMessage.MailStructure()
17.9.2006 Original Post
- I was not too sure how to map the various multipart entities to email body, etc. I analysed probably thousands of emails I received and populated the
RxMailMessage properties as appropriate, but it is very likely that you might receive a different formatted email. Please provide some feedback here, if you do so or find any bugs.
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