Missing e-mails !!

Hi,

Sometimes user sent internet e-mails from outlook2000 installed on a win98 machine and the e-mail appeared in the sent folder but some of them never got to the receipients. This is kind of weird but then e-mail sent from outlook2000 installed on win2k got to the other end. I am just wondering if there is a tracking mechanisim in exchange5.5 that I could use to track the missing e-mail. Could this be the problem with outlook2000 and win98 or something else?

Please help !!

Edmond