Strange...
Does the email client add other characters before the '-- do not edit --', so it becomes something like '>> -- do not edit -- '?
This should not make any difference. The code is (in include/class.mailfetch.php and api/pipe.php)
list($message)=split($tag,$var); where $tag is "-- do not edit --"
That it does not work is indeed strange. However, personally, I recommend not to use this feature anyway because it removes anything after "-- do not edit --" no matter what, including signatures of my customers and whole messages if people use (bottom-posting) or edit the quoted parts (inline replying).
@[deleted]: I suggest you try to get a hold on the source code of the messages (outgoing and incoming) to see why the text "-- do not edit --" is being mangled (or whether it isn't).
