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UUENCODE Attachment Format

The Unix-to-Unix encode (UUENCODE) format provided one of the earliest ways to add attachments to messages. In the UUENCODE format, attachments are appended to the message body after being encoded using the UUENCODE algorithm. Each attachment is prefixed with the file name and the encoding end string. Multiple attachments are individually appended in sequence and separated by a blank line. In the UUENCODE attachment format, the message body consists of only two basic parts the message text and the message attachments.

The following example shows a message body formatted using the UUENCODE attachment format that contains the message text and an attached Microsoft® Word file:

To: "User 1" <user1@example.com>
From: "User 2" <user2@example.com>
Subject: My latest draft.

I've attached my latest draft.
Regards,
User 2

begin 644 mydraft.doc
AAAAAAAAAAFDFAAAAFAFAF....

Although the UUENCODE format provides a way to add attachments to messages, it does not define ways to:

  • Indicate the type of the attachment, except through the file's extension.

  • Specify alternate character encoding for the message text to support international languages.

  • Relate groups of attachments.

  • Indicate that the message text is a form of rich text, such as HTML or Rich Text Format (RTF) formatted text.

  • Provide future enhancements to a structure of complex message bodies. The UUENCODE attachment format is neither flexible nor descriptive.

The next section, MIME Message Format, describes a message format that enables each of these enhancements.