MS outlook97 tracking option

a senario :
– user sent email to 10 persons ( example ) and set the tracking options to be notified after the recipient read the email.
– out of 10 persons, only 2 recipients are not replied with the notification but this 2 reciepients replied her email.
– I believed that the recipients may be doing something at their own PC settings.
– The sender and the receipents are working in the same company and are using Outlook 97.
– Anyone know how the receipent manages to reply the email withoutsending out a notification email to the sender?