MIME parts often contain more than one document within a single part. Each document contained in a multipart part is a complete MIME part or MIME message. Therefore, the MIME specification refers to messages that contain multiple documents as multipart parts. This section describes the step-by-step process for creating a multipart/mixed part, a multipart type typically used for email attachments. The process for other multipart content types is similar.

To create a multipart/mixed message:

Section 9.5, “Program: Creating a Multipart MIME Message,” presents a complete example program that creates a multipart MIME message.

Each part of a multipart is a complete, valid MIME part. Construct MIME parts as described in Section 9.2, “Creating a Simple MIME Part,” above.

Some multipart types require that the parts contain specific headers. For example, a multipart/related message requires each MIME part the message contains to have either a Content-ID or a Content-Location header. See RFC 2387 for details on the multipart/related content type.

Every multipart must contain a Content-Type header. The header must have a boundary string, and the boundary string must match the boundary string in the multipart body of the message. Create a Content-Type header as shown below:

The default constructor for an RWMimeMultipartType creates a multipart/mixed type with a boundary string created by the getUniqueBoundaryString() function of RWMimeUtils. To create a value with a different subtype, provide the subtype to the constructor. For example, the line below creates a multipart/alternative Content-Type value with an automatically-generated boundary parameter:

MIME requires that a message contain a MIME-Version header. Construct an RWMimeVersionHeader.

The Content-Transfer-Encoding for a multipart message must be one of the identity encoded types – either 7bit, 8bit, or binary. To determine the Content-Transfer-Encoding:

MIME parts often require protocol-specific headers in addition to MIME headers. For example, email messages require a Date header and a From header. The RWMimeGenericHeader class represents a general-purpose header. If the part requires headers that aren’t part of the MIME format, use instances of RWMimeGenericHeader for those headers. For example, the line below creates a From header for an email message:

Construct the part as an instance of RWMimeMultipart:

Although a multipart is not required to contain a preamble, most multipart messages that contain human-readable content provide one. Set the preamble using the setPreamble() function:

Use the insertPart() function of RWMimeMultipart to add each MIME part to the multipart:

Add the parts in the order in which they should appear in the multipart.

Add the headers to the part using insertHeader() as shown below:

Remember that a multipart must contain a Content-Type header with a multipart media type, and that every MIME message must contain a MIME-Version header.

The MIME part is now complete and ready to use. To send the message over a network, convert the message into a MIME string using the asString() function, as shown:

The asString() function doesn’t enforce a particular part style, or even guarantee that the part is correct MIME. The function does contain some safeguards that prevent a program from generating an unintelligible message. If the function cannot produce a structurally correct MIME message, the function throws RWMimeError.