Client sent to a user but received other user notification

I have a user (callled ‘A’) which sent a mail to user ‘B’ only but he received return receipt notification not only from user ‘B’ also from user ‘C’. Anyone can help me why it happen?

Thanks

Denny James

Subject: Client sent to a user but received other user notification

Hi;This only happens if the C user is admin or have read rights over the B mail DB. Kindly check if the B Mail DB is not available with C.

Thanks