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.