Index: Mail.php
===================================================================
--- Mail.php	(revision 10901)
+++ Mail.php	(working copy)
@@ -395,9 +395,7 @@
       if (Zend_Mime::isPrintable($value)) {
           return $value;
       } else {
-          $quotedValue = Zend_Mime::encodeQuotedPrintable($value);
-          $quotedValue = str_replace(array('?', ' ', '_'), array('=3F', '=20', '=5F'), $quotedValue);
-          return '=?' . $this->_charset . '?Q?' . $quotedValue . '?=';
+          return Zend_Mime::encodeQuotedPrintableHeader($value, $this->_charset);
       }
     }
 
@@ -613,7 +611,7 @@
     {
         return $this->_subject;
     }
-    
+
     /**
      * Sets Date-header
      *
Index: Mime.php
===================================================================
--- Mime.php	(revision 10901)
+++ Mime.php	(working copy)
@@ -158,6 +158,100 @@
         return $out;
     }
 
+    public static function encodeQuotedPrintableHeader($str,
+        $charset,
+        $lineLength = self::LINELENGTH,
+        $lineEnd = self::LINEEND)
+    {
+        $out = '';
+        $str = str_replace('=', '=3D', $str);
+        $str = str_replace(self::$qpKeys, self::$qpReplaceValues, $str);
+        $str = rtrim($str);
+
+        // Our overriden Tend_Mail::_encodeHeader() method does not do this step
+        // anymore, so we do it here
+        $str = str_replace(array('?', ' ', '_'), array('=3F', '=20', '=5F'), $str);
+
+        // The RFC says that the whole encoded word must not be longer than 75
+        // characters and not only the encoded text. We substract the length
+        // of the charset etc. from the line length to find out, how long the
+        // encoded text can be.
+        $encodedTextLength = $lineLength - strlen('=?' . $charset . '?Q?' . '?=');
+
+        // Split encoded text into separate lines
+        while ($ptr = strlen($str))
+        {
+            // if the rest of $str is still longer than the allowed encoded text length
+            if ($ptr > $encodedTextLength)
+            {
+                $ptr = self::getSplittingOffsetForQuotedPrintableStrings($str, $charset, $encodedTextLength);
+            }
+
+            // Add string and continue
+            // RFC 1522 says, we need CRLF + SPACE between multiple encoded-words
+            $out .= '=?' . $charset . '?Q?' . substr($str, 0, $ptr) . '?=' . $lineEnd . ' ';
+            $str = substr($str, $ptr);
+        }
+
+        $out = rtrim($out, $lineEnd);
+
+        return $out;
+    }
+
+    public static function getSplittingOffsetForQuotedPrintableStrings($str, $charset, $maxLength)
+    {
+        if ($charset == 'utf-8')
+        {
+            $ptr = $maxLength;
+            $pos = strrpos(substr($str, 0, $ptr), '=');
+
+            if ($pos !== false && $pos >= $ptr - 2)
+            {
+                // $pos is at the beginning of an encoded byte.
+
+                // we need the next 2 characters after the '='
+                $ord = hexdec(substr($str, $pos + 1, 2));
+
+                if ($ord >= 192)
+                {
+                    // $pos is at the beginning of an encoded multibyte character
+                    return $pos;
+                }
+                else if ($ord >= 128)
+                {
+                    // $pos is between to bytes of a single multibyte character
+                    // we call this method with $maxLength reduced to $pos-3
+
+                    return self::getSplittingOffsetForQuotedPrintableStrings($str, $charset, $pos - 3);
+                }
+                else
+                {
+                    // This is just a not printable singlebyte character, we can split here
+                    return $pos;
+                }
+            }
+            else
+            {
+                // there are no encoded (multibyte) characters at $ptr
+                return $ptr;
+            }
+        }
+        else
+        {
+            // This works only for singlebyte character sets.
+
+            // Ensure we are not splitting across an encoded character
+            $pos = strrpos(substr($str, 0, $maxLength), '=');
+
+            if ($pos !== false && $pos >= $maxLength - 2)
+            {
+                return $pos;
+            }
+
+            return $maxLength;
+        }
+    }
+
     /**
      * Encode a given string in base64 encoding and break lines
      * according to the maximum linelength.