Email accounts

Hi
Recently posted regarding virus removal and resulting probs. Re-installed XP
and all seems to be OK,but for the email. My neighbour uses Yahoo and
Incredimail to send and recieve. I have set up his account in Outlook
Express using his Yahoo account details but it will only send and not
receive. So, I redo the information and then it recieves but does not send,
so I redo them and it sends but no recieves. what the heck is going on? I
have checked the Yahoo site and see that the settings are:
pop.mail.yahoo.com, and
smtp.mail.yahoo.com
I really am at a loss at to why this swaps back and forth willy-nilly when
the settings are the same after each change.
What could be doing this? How can get over this problem?
Many thanks
Nig