can't read sender message but shows in desktop notification and IS

I have Outlook 2003.
i've only had this problem with one person. i thought i had fixed problem
because i deleted and then re-added his email to receive sender list and
domain.

I was able to receive and read his emails in Outlook after doing above and
suddenly today when he sent me an email with an attachment, i can't read his
responses again?? I have to go my online ISP to read.

I can read his response, however, on the little window that pops up on your
desktop upon receiving email inbox notification.
It’s just not showing up in outlook inbox?????