Subject headers encoded with rfc2047 Q-encoding (=?UTF8?Q?...) are not decoded by many MUA's since the encoding is incorrect. The problem is that ALL spaces and tabs must be encoded in the headers, but dokuwiki uses the same function (mail_quotedprintable_encode) to encode both headers and body. The attached patch fixes this problem.
I have applied the patch to 2006-11-06 and verified that it's working with pine, squirrelmail, and firefox. (Firefox even accepted the incorrect encoding with spaces). I have also tested in 2007-01-15 which has an identical mail.php. In more recent development versions mail.php is updated, but I can't see that the changes has anything to do with this bug.