Call them and ask.
Read receipts and Delivery receipts are options, but, generally, shouldn't
be trusted with 100% certainty. Many clients won't send a Read receipt, and
other clients, like Outlook, give the recipient the option to say No. A
Delivery Receipt generally means that the Server *accepted* the message for
the user. If it's a pop3 account, it doesn't mean that the user has
actually done whatever they needed to do to read it.
If it is entirely within Exchange, turn on message tracking on the Exchange
Server side and you can verify that the message was delivered to the
mailbox.
And no, you asked in a newsgroup, come back to read the answer.
--
f.h.