This is useful to fix annoying issues between certain email senders and email clients.
The email can be sent through procmail, and then a more advanced language can be used to parse the email, modify it and save resend it.

PHP

• PEAR mimeDecode can be used to decode the mime message:
  • http://www.theukwebdesigncompany.com/articles/pear-mimedecode-module.php
  • http://pear.php.net/package/Mail_mimeDecode/docs/latest/Mail_Mime/Mail_mimeDecode.html
• PEAR mime can be used to re-encode the message

Other links:

• http://www.webmasterworld.com/webmaster/3359450.htm

Issues:
These libraries don't do round trips very well. The structure created by mimeDecode cannot just be modified and turned back into a string again.

Notes:
for testing command line php scripts the following error handler is useful to see errors:

  // A custom error handler                                                                             
function CliErrorHandler($errno, $errstr, $errfile, $errline) {
  fwrite(STDERR,"$errstr in $errfile on $errline\n");
}
// Tell PHP to use the error handler                                                                    
set_error_handler('CliErrorHandler');

Ruby

The Rmail library looks better than the php one. It seems like it would be easy to go from mime message to a struct and back again. http://www.rfc20.org/rubymail/
There is also a rubyfilter library which uses rmail which might be a replacement for procmail. However it seems like it isn't maintained much anymore. The main website is missing.
This page has some more details: http://www.securityfocus.com/tools/3029