E-Mail File Attachments Using MIME

The classes presented in this article extend those described
in "SMTP MFC Classes" to handle messages using the
Multipurpose Internet Mail Extensions (MIME) protocol. MIME is an
e-mail message protocol (described in RFCs 1521 and 1522) that
allows messages to incorporate non-text content while still
remaining compliant with RFC 822.

This non-text content could be embedded audio or video, an
"external" message (meaning the content is not stored
in the message itself, but at an external location), or one or
more binary file attachments. Not all of these content types are
supported by the classes in this article, but you can easily
extend them to do so.

How MIME Works

RFC 822 describes a generic message format, and specifies some
standard header information. It says almost nothing about the
body of the messages, except that it is a chunk of ASCII text.

MIME takes advantage of that by superimposing a format for the
body text, and it also defines some header lines of its own. The
neat thing is that since the body is still just a chunk of text,
it doesn’t break any rules established by RFC 822. This means
that any RFC 822-compliant transport system can handle MIME
messages without modification. The transport system expects a
message to have a chunk of 7-bit text, and MIME messages happily
oblige them. The transport system neither knows, nor has reason
to care, that lines 50 through 700 of the text chunk actually
represent an executable file. For that matter, it doesn’t care
that the chunk of text in a non-MIME message represents the
English sentence "Attack at dawn". As far as a mail
server transporting the message is concerned, it’s all just a
bunch of 7-bit characters to send up the chain.

Of course, the receiving clients of the message do
care because that chunk of text has to be converted back into a
binary file. They can do this only if they know about the format
of the message, i.e., only if they are MIME-compliant mail
readers.

MIME Headers

MIME defines five additional header lines that inform the
receiving client about how the body should be interpreted.

Header Meaning

Content-Type

Specifies the type
of data contained in the message
Content-Transfer-Encoding Specifies how the data is converted
into 7-bit text
MIME-Version Indicates the MIME
compliance level to which the message is encoded.
Content-ID Uniquely identifies the body. This is
used for splitting the contents of large messages into
smaller messages.
Content-Description Ignored by MIME
applications. Gives a human reader an indication of the
content.

The Content-Type header tells decoders what
kind of data is contained in the body, giving both a broad data
type (such as "image"), and a specific type (such as
"GIF") separated by a forward slash ("/").
Following the type information, the Content-Type header may
include additional parameters in name=value pairs, each
separated by a semi-colon (";"). What these additional
parameters are depends on the type of data.

Everyone, and their mothers, and their mothers’ dogs, are
proposing new content types (along with their parameters) all the
time, so you’ll have to go swimming in an ocean of standards
documents to discover them all.

Example:

Content-Type: text/plain; charset="iso-8859-1"

The more content types your mail reader can handle, the more
capable it is. Outlook supports text/html, so it can
display HTML messages to the user directly. It supports image/jpeg,
so it can show you pictures right there in the message.

Any content type that the mail reader can’t support itself
should be saved to disk so that the user can open it with
something that knows how to handle it.

In other words, there’s no such thing as a "file
attachment"; there are only content types that are encoded
and decoded. What you do after decoding is up to you. Do you
support application/zip? Then unzip the binary block. If
not, save it to disk and let someone think of it as a "file
attachment".

What most people think of as a file attachment is usually a
body of type application/octet-stream. That signifies a
chunk of 8-bit bytes… but that could be anything! Without more
concrete information about the data, there’s really nothing the
mail reader can do except save it to disk.

The Content-Transfer-Encoding header
identifies the mechanism a decoder should use in order to convert
the message body back into its original form.

The biggies are 7bit, 8bit, Binary, Quoted-Printable,
and Base64. Regardless of what content types your mail
reader supports, you need to be able to decode the message in the
first place. You don’t have to support image/gif, but
you need to be able to turn the body into a GIF file that you can
save to disk. Your program can’t do this unless it handles all
encoding mechanisms (you can’t control how someone else’s program
encoded it, so you need to handle them all). You can get away
with supporting only text/plain because you can just
dump other types of data to disk, but you can’t do anything if
you can’t read the data to begin with.

Luckily, only Base64 encoding could be considered even
slightly difficult to implement.

Multi-Part Messages

MIME completes the illusion of file attachments by allowing
the message body to be divided into distinct parts, each with
their own headers. The content type multipart/mixed
means that the content of the body is divided into blocks
separated by "–" + a unique string guaranteed to not
be found anywhere else in the message. If you say that your
boundary string is "MyBoundaryString", then all
occurrences of that string will be treated as a boundary. So it
better not be in the message the user typed or it won’t be
decoded correctly.

The boundary string is specified as a parameter to the
Content-Type header:

Content-Type: multipart/mixed; boundary="MyBoundaryString"

Do not include the preceding "–" in the value. The
parts should then be separated with this:

--MyBoundaryString

And the end of the entire message will be indicated with a
trailing "–" as well:

--MyBoundaryString--

Text before the first boundary and after the end-of-message
boundary is ignored by decoders, but since a non-MIME reader will
simply display the whole thing as text, these areas can be used
to tell the user to get a better mail reader.

From: [email protected]
To: [email protected]
Subject: In One Ear and Out Your Mother
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="MyBoundaryString"
This is a MIME message. If the next few lines look like gibberish,
then your mail reader sucks.
If you are using a MIME reader, then you aren't even seeing this.
--MyBoundaryString
Content-Type: text/plain
Content-Transfer-Encoding: 7bit
The charset= parameter is omitted and will default to US-ASCII.
In fact, I could have had a blank line as my header and it would have
defaulted to exactly what I specified on those header lines.
--MyBoundaryString
Content-Type: application/octet-stream; file="ATTACHMENT.EXE"
Content-Transfer-Encoding: base64
AxfhfujropadladnggnfjgwsaiubvnmkadiuhterqHJSFfuAjkfhrqpeorLAkFn
jNfhgt7Fjd9dfkliodf==
--MyBoundaryString--
This text is ignored.

Note that each part of the message conforms to RFC 822,
including the "sub" headers.


The MIME Classes

At the application level, these classes are simple to use:

  1. Create a message object of type CMIMEMessage
  2. Populate it
  3. Call AddMIMEPart() passing the path to a file you want to
    attach (one call per file).
  4. Create a CSMTP object and connect to a server
  5. Call the CSMTP object’s SendMessage(), giving it the
    message object.
  6. Disconnect from the server

As is, you can send text messages with one or more reasonably
sized file attachments. But since MIME is constantly evolving,
and you may need to support various character sets, you may need
to extend these classes at some point. So here’s how it all
works.

CMIMEMessage

This is where all the action takes place. It is derived from
CMailMessage and works closely with its base class. Conceptually,
CMailMessage represents a message that conforms to RFC 822, and
since MIME merely adds some headers and formats the body,
CMIMEMessage should only do the same. CMailMessage does what it
needs to do to make the message RFC 822-compliant; CMIMEMessage
only tacks on additional information to make the message
MIME-compliant.

CMIMEMessages are themselves made up of distinct parts– each
with their own content types, parameters and encoding. Instead of
a monolithic class that handles all content types, I decided that
it should know nothing about content types at all. In its
constructor, it registers as many content "agents" as
required. Each agent is identified with a code. Message-parts are
added individually to an internal list along with the code of the
agent that knows how to handle them. When CMIMEMessage needs to
build its body, it asks the appropriate agent to actually
incorporate the part into the body. In this way, whenever I need
to handle a new MIME content type, I create an agent by deriving
from CMIMEContentAgent, and register the agent object in the
constructor of a CMIMEMessage-derived class.

There are two content agents in this package: CAppOctetStream
and CTextPlain. These classes (both derived from
CMIMEContentAgent) know how to handle the
"application/octet-stream" and "text/plain"
content types, respectively.

The message-parts and agents are kept in sync with a unique
code. Agent objects are given their code when constructed. When
message-parts are added to the message, this same code is used.
The codes are defined as a public enum, CMIMEMessage::eMIMETypeCode.
Because the message class controls the codes, it’s very easy to
keep things in sync. Note the final item,
"NEXT_FREE_MIME_CODE". When you derive a message class
(and agents) to handle other content types, begin your new codes
with this one. When you register your new agents, give them one
of your new codes.

Example:

Say you want to create a message class that handles the
"application/zip" content type.

You’d first derive a CMIMEContentAgent class that does the
useful work. Then you’d derive a message class from CMIMEMessage
and create the appropriate code like this:

class CEnhancedMIMEMessage::CMIMEMessage
{
public:
	enum eMIMETypeCode
	{
		APP_ZIP = CMIMEMessage::NEXT_FREE_MIME_CODE,
		NEXT_FREE_MIME_CODE  // Ready to use by 4th-generation class.
	};
};

In the constructor, you’d create the new agent using this code
and register it. You would also use this new code when you add
message-parts.

You can continue this indefinitely– each sub-class adding the
ability to handle more content types.

The internal class CMIMETypeManager, manages
the agents. It’s job is to cough up an appropriate agent for a
given content type and to delete the agent objects.

CMailMessage::FormatMessage() is what gets
the ball rolling. (It is called by CSMTP::SendMessage(), but you
you can call it yourself whenever you want.) There is some
dancing back and forth between base class and derived class
that’s not obvious, and you should be aware of it. CMailMessage::prepare_header()
is virtual, so FormatMessage() will call the derived class’s
version. The first thing the derived class’s version does is
call the base class’s version
so that it can do whatever it
is supposed to do to the header. In this way each class only
worries about its own header lines. (Classes derived from
CMIMEMessage don’t need to override prepare_header() because they
are CMIMEMessages, and that class knows how to prepare
its headers.)

CMIMEMessage::AddMIMEPart() adds
message-parts to the list of parts used to construct the body. It
takes the code of the agent, a suggested encoding type,
the actual content, and a flag argument that indicated whether
the content should be treated as a path or as actual content. The
default parameters supplied are sufficient to attach a file.

msg.AddMIMEPart( "C:AUTOEXEC.BAT" ); // Send someone my autoexec file	

It’s not only for files, but for any message-part (including
what’s normally thought of as the "body"). Indeed, if
you assign the m_sBody member directly (which is perfectly
acceptable), it will get converted into a message-part to be
processed along with the others. Eventually, m_sBody will be a
long string of text incorporating everything in the message.

Limitations and Warnings

CMIMEMessage class doesn’t support message
splitting. So if your mail server has a size limit, and doesn’t
break apart messages itself, you’ll need to derive a class to
handle this. This will involve deriving from CSMTP as well.

CMIMEMessage builds the message body in memory.
If you attach a 100-meg file, it will consume 100 megs of memory,
plus overhead while processing (and I use CStrings extensively).

These two limitations mean that there is an
undefinable size limit for file attachments (undefinable because
it depends on your server and the memory manager used by your
OS).

One of the first enhancements I did was to
override CSMTP::SendMessage() to poll a
CMailMessage::FormatMessage()-like function that returned a
status code to indicate its next course of action. CREATE_NEW
indicated that the CSMTP object should tell the server it’s done
with this message and is going to send another one. Also, the
function accepted a buffer and the message was built, one line
per call, into that buffer instead of into m_sHeader and m_sBody.
I can think of 3 other ways to handle this situation, and I’m
sure you can too. Heck, you could even break encapsulation and
just pass a pointer to the CSocket the SMTP object is using. Let
the message object pump its own data to the mail server.

Using the Classes in Your Projects

I know some of you are using the classes from "SMTP MFC
Classes" in your applications. The implementations of those
classes have changed, but the public interfaces are the same. So,
you won’t need to redo your source code at all. You will just
need to compile it with the new source code from this package
instead.

Use folder names when you extract the package. You’ll find a
folder called SMTP that contains component files (.OGX) that can
be inserted right into your project.

If you have suggestions, find any bugs, or improve these
classes, feel free to contact
me
. In particular, if you develop new content type agents,
I’d love to get my grubby hands on them.

Download the project


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