Why is some of my mail delayed? Some up to 5 days!?

Sometimes it takes 20 seconds, other times it may take 5 days to receive mail. Can't pinpoint a particular contact, sometimes it goes directly to my yahoo address, sometimes through my company address and then to my yahoo address. It sounds like I'm not the only one...is there a professional opinion, or better yet, a Yahoo techie that can answer/fix this problem? I rely on my email in my business (as do millions of people, I'm sure) and cannot afford delays like this!!!! Help!