Ali,
MSP receipt handling allows at least two _policies_.
The first is one where the receipt is generated prior to the recipient
reading the mail. In this case, the MSP processing software generates
the receipt and waits for a successful submission response from the MTS
before passing the content to the recipient's viewer. Note that the
control is in the implementation, not in the cryptography or the protocol.
The second allows the recipient to either explicitly or implictly
control receipt return. e.g. the software presents a "Return receipt ?"
popup or the user "reboots" their system prior to receipt return but
after viewing the message.
Note that both of these policies are represented in different segments
of the government community and both are valid for those community.
I would suggest that similar policies exist in the business community.
Note further that the syntax and protocol of receipt handling (e.g.
when and whether a receipt token is returned to the requester) has
_nothing_ to do with the semantics. If the original message was
ambiguous then the receipt may still be meaningless. (That's why
we have lawyers :-) ) If the receipt is time-context sensitive,
then the lack of a trusted third-party timestamp on the receipt
may also allow repudiation.
John Lowry