[Exim] returned message size

Hello,
I'm running exim 3.16 on ICL UNIX. Works nice.
Still, it doesn't work entirely like I would like ...
I set

message_size_limit=2000K
returned_size_limit=500K

Messages larger than 2000K are not accepted, and the sender gets the message
saying that message size is above the limit. Sender should also get only
first 500K of his original message (as set in returned_size_limit), but he
gets the entire (sometimes Very Large) message.
If returned_size_limit is not defined, it behaves exactly the same. It seems
returned_size_limit (that should be 100K by default) has no influence on
anything. Such behaviour is a waste of bandwidth.

What am I missing here?

TIA,
Hana Breyer Priselac
sysadmin