Don't set overall encoding just because we're changing a sub-part - quoted-printable and base64 are not valid encodings for a multipart container anyway.

Author: Synchro

class.phpmailer.php

@@ -2130,12 +2130,12 @@ public function createBody()
         //Can we do a 7-bit downgrade?
         if ($bodyEncoding == '8bit' and !$this->has8bitChars($this->Body)) {
             $bodyEncoding = '7bit';
+            //All ISO 8859, Windows codepage and UTF-8 charsets are ascii compatible up to 7-bit
             $bodyCharSet = 'us-ascii';
         }
         //If lines are too long, and we're not already using an encoding that will shorten them,
-        //change to quoted-printable transfer encoding
+        //change to quoted-printable transfer encoding for the body part only
         if ('base64' != $this->Encoding and self::hasLineLongerThanMax($this->Body)) {
-            $this->Encoding = 'quoted-printable';
             $bodyEncoding = 'quoted-printable';
         }
 
@@ -2144,10 +2144,11 @@ public function createBody()
         //Can we do a 7-bit downgrade?
         if ($altBodyEncoding == '8bit' and !$this->has8bitChars($this->AltBody)) {
             $altBodyEncoding = '7bit';
+            //All ISO 8859, Windows codepage and UTF-8 charsets are ascii compatible up to 7-bit
             $altBodyCharSet = 'us-ascii';
         }
         //If lines are too long, and we're not already using an encoding that will shorten them,
-        //change to quoted-printable transfer encoding
+        //change to quoted-printable transfer encoding for the alt body part only
         if ('base64' != $altBodyEncoding and self::hasLineLongerThanMax($this->AltBody)) {
             $altBodyEncoding = 'quoted-printable';
         }