Need help with email setup

I am having trouble with email setup on my iphone3g. I have tried about every combination that i can think of but nothing seems to work. I have 3 email accounts that i am trying to set up. One is a free yahoo account & one is a typical pop account & one, which is my work email uses microsoft exchange. For the host name,incoming & outgoing server i have the same name as on my home computer. I have my full email address as my username & i enter my password. The only thing i can think of trying is to sync with my home computer but i thought this was going to be much easier. Yahoo email gives me an error that says the imap server imap.apple.mail.yahoo.com is not responding