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Netmime Tutorial

Structure of Mail Messages

Nowadays mail messages are in MIME format. This format allows us to attach files to messages, and to encode the main text in markup languages like HTML. In principle, mail messages have only one header block (with fields like "Subject", sender and receiver addresses, etc.) and one body block. However, this is only one view on the mail format, e.g. as seen by MTAs (mail transfer agents). The MIME format adds the possibility to structure the body block into "parts" by additional encoding sequences. The MTAs can simply ignore this additional stuff, but software creating and analyzing mails can usually not. In Netmime, one can control whether one wants to see the parts or not.

Logically, the parts of the mail body are small mail messages themselves. This means that every part has again a header and a body. The header can, in principal, contain any number of fields, and any kind of field, but in practice only a small subset of the possible fields are used, in particular only those fields that are necessary to describe the body of the part. The body can be a normal text or data block, but it is explicitly also allowed that the body is again structured into a sequence of parts. Thus complex mail messages are recursive data structures (to be exact, they are trees).

For example, a message with two attachments usually looks like:

  (mail_header, mail_body)
                 +-- (main_text_header, main_text_body)
                 +-- (att1_header, att1_body)
                 +-- (att2_header, att2_body)

The headers contains two crucial fields that control the structure of the message:

  • The Content-type describes the kind of data found in the body, e.g. "text/html". When the Content-type has the major type "multipart" (e.g. "multipart/mixed"), the body is composed of subparts. For all other types, the body is a leaf of the message tree. (To be exact, there is another major type that opens a further dimension of "message-in-message" composition: "message". This type is usually used when it is not clear whether the inner message is syntactically correct. Netmime handles this type always as leaf, but users of Netmime can try to parse these inner messages themselves.)
  • The Content-transfer-encoding describes how the body data is encoded as ASCII text. It is usually only set for leaves. Recommended values are "quoted-printable" for bodies that contain some kind of ASCII text, and "base64" for binary data.

Messages in Netmime

In Netmime, the types of mail headers and mail bodies are defined before and independent of their implementations: We have the types

  • class type mime_header: Specification of possible header implementations
  • class type mime_body: Specification of possible body implementations
  • type complex_mime_message: The type of a message tree
and the implementations

  • class basic_mime_header: A basic header implementation
  • class memory_mime_body: A body implementation storing the contents in an O'Caml string in-memory
  • class file_mime_body: A second body implementation storing the contents in an external file
Of course, the implementation classes fulfill the specifications of the corresponding class types. For completeness, there are also reduced read-only class types that maybe helpful for signatures to indicate that a function does not modify a header or body. In principal, one can also define further implementations provided they fit to the class types.

The type complex_mime_message represents the message as a tree. We have:

type complex_mime_message = mime_header * complex_mime_body
and complex_mime_body =
  [ `Body of mime_body
  | `Parts of complex_mime_message list
For example, the above mentioned mail with two attachments has the following representation:

let tree =
  (mail_header, `Parts [ (main_text_header, `Body main_text_body);
                         (att1_header, `Body att1_body);
                         (att2_header, `Body att2_body) ] )

Here, *_header are objects of type mime_header, and *_body are objects of type mime_body. It is obvious how to create the tree once one has these objects: Just use the syntax in this expression. Beginners of O'Caml should recall that it is as easy to decompose such structured values by using the pattern matching feature of the language. For example, to get the main_text_header of tree, use

let main_text_header =
  match tree with
      (_, `Parts ( (mth, _) :: _ )) -> mth
    | _ -> failwith "Message has unexpected structure"

(Note that [x1;x2;...] is just an abbreviation for x1 :: x2 :: ... :: [] ; by switching to the "::" syntax the message may have any number of parts in order to be matching.) At the first glance, it looks a bit strange to access the inner parts of mail messages in this way, but pattern matching is a very powerful sword once one gets accustomed to it.

Another hint: Because complex_mime_message is a quite challanging type for the compiler, it is often necessary to give type annotations, such as

(tree : complex_mime_message)

before passing such values to functions, otherwise you get compiler errors.

Accessing Headers

It is easy to get and set the fields of headers, e.g. mail_header # field "subject" returns the "Subject" header field as string (or raises Not_found). The names of header fields are case-insensitive. To set a field, use update_field, e.g. mail_header # update_field "subject" "Ocamlnet is great" .

The methods field and update_field process the field value as unparsed string (the parsers do only very little preprocessing, e.g. one can configure to remove all linefeeds). The module Mimestring has a lot functions to parse and generate field values with a certain syntax. For example, "Subject" may contain so-called encoded words to express text written in a character set other than ASCII. To parse this, use

let subject = mail_header # field "subject" in
let word_list = Mimestring.scan_encoded_text_value subject in
Now, the words contained in word_list can be accessed with a number of functions, e.g.
let word_val = Mimestring.get_decoded_word word in
let word_cset = Mimestring.get_charset word
Here, the string word_val is the word written in the character set word_cset.

For example, for the "Subject" field


this method returns a word_list with one word, and for this word word_val = "this is some text" and word_cset = "iso-8859-1".

To create such structured header values, there is the function write_value in Mimestring. This function requires some more background beyond the scope of this tutorial. As this function also supports folding of header fields, we explain only this particular application.

Folding means that long header values must be split into several lines. There is a soft limit of 78 bytes and a hard limit of 998 bytes (not counting the end-of-line sequence). The soft limit only ensures that values can be displayed in usual terminals or windows without needing horizontal scrolling. Values exceeding the hard limit may be truncated in mail transport, however. To fold a string s composed of words, first split it into its words, make atoms of them, format them with write_value, and put the result into the header field (note: this example can be programmed better, see below):

let name = "Subject" in
let words = Str.split (Str.regexp "[ \t]+") s in
let atoms = (fun w -> Mimestring.Atom w) in
let buf = Buffer.create 100 in
let ch = new Netchannels.output_buffer buf in
  ~maxlen1:(78 - String.length name - 2)
  ~hardmaxlen1:(998 - String.length name - 2)
mail_header # update_field name (Buffer.contents buf)

Unfortunately, there is no general method that can fold any kind of string. The problem is that folding is only allowed at certain places in the string, but this depends on the type of the header field. The shown method works only for informational texts like "Subject". For other fields, like "Received", the method would have to be varied, especially how the list atoms is determined. The syntax of the field must be known to compute atoms.

In the module Netsendmail you can find formatting and folding functions for informational texts like "Subject", and for mail addresses. With these functions, the "Subject" field could also be set by

let atoms = Netsendmail.create_text_tokens s in
mail_header # update_field 
  name (Netsendmail.format_field_value name atoms)

Accessing Bodies

Both types of bodies (in-memory, and file) support the following two ways of accessing:

  • Get/set the value as O'Caml string
  • Read/write the value as object channel (see Netchannels)
Note that when the value of a file-based body is changed, the file is overwritten, independently of which of the two ways is taken.

The string access is very simple: To get the value, just call value:

let s = body # value

To set the value, just call set_value:

body # set_value s

The string returned by value is not transfer-encoded, or better, all such encodings (e.g. BASE-64) are decoded. Of course, set_value expects that the passed string is not decoded, too.

Note that using value may be dangerous (or even fail) when the body is stored in a file and is very large. value forces that the file is completely read into memory. You may run into serious problems when there is not enough memory, or when the value is larger than Sys.max_string_length (16MB on 32 bit platforms).

Fortunately, there is the channel-based access method. It does not need much memory, even when large bodies are accessed. However, one does not get access to the completely body at once, but only chunk by chunk. For example, to read a body line by line, use:

let ch = body # open_value_rd() in
let line1 = ch # input_line() in
let line2 = ch # input_line() in
ch # close_in()

As for value, there are no transfer encodings in the returned lines.

The channel ch can be used whereever an Ocamlnet function allows it, i.e. it is a full implementation. For example, one can pass it to the HTML parser:

let ch = body # open_value_rd() in
let html_doc = Nethtml.parse ch in
ch # close_in()

To set the value using a channel, a body can also be opened for writing:

let ch = body # open_value_wr() in
ch # output_string "First line\n";
ch # output_string "Second line\n";
ch # close_out()

Parsing mail messages

The message to parse must be available as an object channel. Recall that you can create an object channel from a string with

let ch = new Netchannels.input_string s

and from a file with

let ch = new Netchannels.input_channel (open_in "filename")

so one can parse mail messages coming from any source. As only sequential access is needed, it is even possible to read directly from a Unix pipe.

Now, it is required to create a so-called netstream from ch:

let nstr = new Netstream.input_stream ch

A netstream is an object channel with additional look-ahead features. We need it here because the parser can then recognize certain patterns in the message in a simpler manner, for example the escape sequences separating the parts of a structured body.

Finally, one can invoke the parser:

let tree = read_mime_message nstr

There are a number of optional arguments for this function that can modify the way the message tree is generated. By default, all bodies are created in memory, and the tree is deeply parsed (i.e. inner multipart bodies are represented in tree form).

When bodies should be written to disk, the argument storage_style can be passed: It is a function that is called whenever a header has been parsed, but before the corresponding body. The function must return the body object for representation and the output channel connected to the body object. For example, to write the bodies into numbered files:

let n = ref 1
let ext_storage_style header =
  let body = new file_mime_body ("file" ^ string_of_int !n) in
  incr n;
  (body, body#open_out_wr())
let tree = read_mime_message ~storage_style:ext_storage_style nstr 

There is also the auxiliary function storage to create such a storage style argument.

The header can be used to generate the file name from it. Often, the filename argument of the Content-disposition field is the original file name before the attachment was appended to the mail message. To get this name:

let filename =
    let disp, disp_params = header # content_disposition() in
    (* disp is usually "attachment", but we don't check *)
    List.assoc "filename" disp_params
    Not_found ->
       ...  (* No such paramater, use other method to gen filename *)

It is usually a good idea to check for dangerous characters in this name ("/", "..") before constructing the name of the disk file.

A final remark: Don't forget to close nstr after parsing (this implicitly closes ch).

Creating Mail Messages

For simple applications, the Netsendmail module has a Netsendmail.compose function. It can create a mail message with attachments, and performs all the encoding details. This function is well explained in its module mli.

Of course, you can also do this yourself: Create the required headers and bodies, and put them together to the resulting tree.


let date =
  Netdate.mk_mail_date ~zone:Netdate.localzone (Unix.time()) in
let mail_header =
  new basic_mime_header [ "MIME-version", "1.0";
                          "Subject", "Sample mail";
                          "To", "recipient\";
                          "From", "sender\";
                          "Date", date;
                          "Content-type", "multipart/mixed" ] in
let main_text_header =
  new basic_mime_header [ "Content-type", "text/plain;charset=ISO-8859-1";
                          "Content-transfer-encoding", "quoted-printable";
                        ] in
let main_text_body =
  new memory_mime_body "Hello world!\nThis is a sample mail.\n" in
let att_header =
  new basic_mime_header [ "Content-type", "image/jpeg";
                          "Content-transfer-encoding", "base64";
                          "Content-disposition", "inline;description=\"My photo\"";
                        ] in
let att_body =
  new file_mime_body "photo.jpeg" in
let tree =
  (mail_header, `Parts [ (main_text_header, `Body main_text_body);
                         (att_header, `Body att_body) ] )

Printing Mail Messages

In order to print tree to the object channel ch, simply call

write_mime_message ch tree

Before invoking this function, ensure the following:

  • The Content-type field of all leaves should be set
  • The Content-transfer-encoding field of all leaves should be set (in doubt use "base64"; if missing, the default is "7bit" - probably not what you want)
  • The Content-type field of multipart nodes should be set (it defaults to "multipart/mixed" if missing)
  • The Content-transfer-encoding fields of multipart nodes should not be set - this is done by the function
If the boundary parameter is missing, the function will invent one; you don't need to deal with this.

The MIME message is written according to the found transfer encodings and the multi-part boundaries.

Don't forget to close ch after writing!

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