Outlook email
The native Microsoft Outlook (2010) email client seems to have difficulty with email encoded as quoted printable with long lines. It inserts spurious line endings for the soft line break (= followed by CRLF). If one enables automatic line breaks in Outlook, these get removed, but a warning is shown and extra white space is added within the word. I found a work-around (or kludge, if you prefer): break lines only at linear white space characters (0x20), as Apple Mail appears to do. I had to modify the PHP Pear mimePart.php function to make this work.