Index: t3lib/class.t3lib_div.php
===================================================================
--- t3lib/class.t3lib_div.php	(revision 5723)
+++ t3lib/class.t3lib_div.php	(working copy)
@@ -5122,7 +5122,14 @@
 				default:
 					$qpValue = t3lib_div::quoted_printable($part,1000);
 					if ($part!=$qpValue)	{
-						$qpValue = str_replace(' ','_',$qpValue);	// Encoded words in the header should not contain non-encoded spaces. "_" is a shortcut for "=20". See RFC 2047 for details.
+						/**
+						 * Encoded words in the header should not contain non-encoded:
+						 *  - spaces. "_" is a shortcut for "=20". See RFC 2047 for details.
+						 *  - question mark. See RFC 1342 (http://tools.ietf.org/html/rfc1342)
+						 */
+						$search = array(' ','?');
+						$replace = array('_','=3F');
+						$qpValue = str_replace($search, $replace, $qpValue);
 						$part = '=?'.$charset.'?Q?'.$qpValue.'?=';
 					}
 				break;