No need for the padding since quoted lines are not wrapped at all

program/steps/mail/func.inc

@@ -1176,7 +1176,7 @@ function rcmail_address_string($input, $max=null, $linked=false, $addicon=null)
 function rcmail_wrap_and_quote($text, $length = 72)
 {
   // Rebuild the message body with a maximum of $max chars, while keeping quoted message.
-  $max = min(78, $length + 8);
+  $max = min(77, $length + 8);
   $lines = preg_split('/\r?\n/', trim($text));
   $out = '';
 

program/steps/mail/sendmail.inc

@@ -448,7 +448,7 @@ else {
   if (empty($_SESSION['compose']['reply_msgid']) && ($flowed = $RCMAIL->config->get('send_format_flowed', true)))
     $message_body = rcube_message::format_flowed($message_body, $LINE_LENGTH);
   else
-    $message_body = rc_wordwrap($message_body, min(79, $LINE_LENGTH + 8), "\r\n");  // +8: be generous with quoted lines
+    $message_body = rc_wordwrap($message_body, $LINE_LENGTH, "\r\n");
   
   $message_body = wordwrap($message_body, 998, "\r\n", true);
   if (!strlen($message_body)) {