I would like to know why someone sending me an email from a Yahoo account is not being received by my outlook 2003 account?

There's a whole bunch of reason I can think of off the top of my head.
1/ Incorrect address
2/ Tagged as Junk Mail in Outlook
3/ Blocked by your mail server
4/ Too large for your mail server to handle
5/ Wrong attachments like EXE being blocked
6/ Outlook's having a bad day, hey it is made my Micro$oft