--- Mail/mime.php	2020-06-27 18:35:27.000000000 +1000
+++ Mail/mime.php	2020-07-25 08:36:14.570334621 +1000
@@ -1527,6 +1527,9 @@
             $this->{$type} .= $data;
         }
 
+        // wordwrap the txtbody to be 750 characters to comply with RFC 2821
+        // CRM-3133
+        $this->{$type} = wordwrap($this->{$type}, 750);
         return true;
     }