For those of you who are new to Windows Phone setting up Yahoo! Mail is an easy process but here is a how to Windows Phone guide to the steps required to get the task done as well as a few other ‘advanced’ features that will make your life simpler.
To set up a Yahoo! Mail account
1.On Start, flick left to the App list, tap Settings , and then tap Email & accounts.
2.Tap Add an account > Yahoo! Mail.
3.Tap the Email address box, and then type your Yahoo! ID.
4.Tap the Password box, and then type your Yahoo! password.
5.Tap Sign in.
Your Yahoo! Mail email will be synced to your phone (without the need of worrying about which pop/smtp mail servers to point to).
For those of you like me who may have more than one Yahoo! Mail account you can add those as well by simply following steps 1-5 above for each additional account.
Once the additional account is added you can set up different sync settings per account, link the additional mailboxes (Yahoo!, Outlook, Hotmail, et.c) so all mail shows up as one thread regardless of who it is sent to (saves a lot of time jumping from one mailbox to another).
Hopefully this information has been helpful and will get you off to a good start syncing all of your Yahoo! Mail.