3.9 decodemail – Decode multipart messages

The decodemail utility is a filter program that reads messages from the input mailbox, decodes “textual” parts of each multipart message from a base64- or quoted-printable encoding to an 8-bit or 7-bit transfer encoding, and stores the processed messages in the output mailbox. All messages from the input mailbox are stored in the output, regardless of whether a change was made.

The message parts deemed to be textual are those whose ‘Content-Type’ header matches a predefined, or user-defined, mime type pattern. In addition, encoded pieces of the ‘From:’, ‘To:’, ‘Subject:’, etc., headers are decoded.

For example, decodemail makes this transformation:

Subject: =?utf-8?Q?The=20Baroque=20Enquirer=20|=20July=202020?=
⇒ Subject: The Baroque Enquirer | July 2020

The built-in list of textual content type patterns is:

text/*
application/*shell
application/shellscript
*/x-csrc
*/x-csource
*/x-diff
*/x-patch
*/x-perl
*/x-php
*/x-python
*/x-sh

These strings are matched as shell globbing patterns (see glob in glob(7) manual page).

More patterns can be added to this list using the mime.text-type configuration statement. See mime statement, for a detailed discussion, and the configuration section below for a simple example.

When processing old mesages you may encounter ‘Content-Type’ headers whose value contains only type, but no subtype. To match such headers, use the pattern without ‘/whatever’ part. E.g. ‘text/*’ matches ‘text/plain’ and ‘text/html’, but does not match ‘text’. On the other hand, ‘t*xt’ does not match ‘text/plain’, but does match ‘text’.

Optionally, the decoded parts can be converted to another character set. By default, the character set is not changed.