Outlook Express 5 Problems

I have a G4 with Microsoft Office and Outlook Express 5. I am finding that
my Outlook Express is not receiving email from certain private domains and I
have to pick up those messages from my Yahoo web-based email. I've checked
the spam filter to see if any of these domains are blocked, but they're not.
What am I missing here? It's a huge problem because I do business with
several companies that have their own email servers, but their messages are
not coming through on my Outlook Express. Thanks.

John