I recently purchased the Nokia Lumia 920 with Windows 8 Phone. Like many others, I was having trouble synchronising my Yahoo mail account on the phone, even though my other mail accounts (Gmail and Hotmail) had synced perfectly well.
I have now discovered that the problem was that my Yahoo account was not set up to enable email to be received on other accounts/devices via POP.
Accordingly, I signed into my Yahoo mail account and took the following steps:
Clicked on Options
Clicked on Mail Options
Clicked on POP & Forwarding
Checked the circle next to Access Yahoo Mail via POP
Clicked on Save
My Yahoo account now syncs with my smartphone.
This was not my original discovery - I owe this solution to an answer in another thread I stumbled across. However, since I had trouble finding that thread (indeed, I only found it by chance), and the answer itself was only one reply in a longer thread, I thought I'd post the solution in a new thread in the hope it would prove useful to others.