Lots of confusion on Yahoo Mail, so here is a quick guide. (Caveat this is for UK Users)

I'm only going to focus on POP and IMAP access here, as Yahoo via Nokia Messaging is easy to set up. If you have problems I have included the account settings at the bottom for you to check against.

1. Decide if you want POP or IMAP access.
2. Go into Email -> New Account , Wizard will start up.
3. Select Next
4. Select Region (United Kingdom)
5. Select Service Provider (Other)

Scroll down the list past .Mac to the bottom where you will see
Yahoo!Mail
Yahoo! Mail Plus
Yahoo UK

-Select Yahoo!Mail if you want IMAP access
-Select YahooMail Plus if you have paid account
-Select Yahoo UK if you want POP access

press Done

Now Enter details for your account

Name : Your Name
User Name: yourname@yahoo.co.uk
Password : you know what to do
email address: yourname@yahoo.co.uk

The wizard will now create the account and you are done. Make sure you do send and receive to sync your emails. This is not a push email service. If you want push email use Nokia Messaging.

For reference here are the server details that get populated by the wizard.

Yahoo!Mail IMAP Servers
==========================
Account title: Yahoo! Mail
Name: Your Name
User Name: youremail@yahoo.co.uk
Password:you know what to do
E-mail address:your email address@yahoo.co.uk
Incoming server *(IMAP4) imap.n.mail.yahoo.com
secure authentication : unchecked
secure connection :None
Port:143
Outgoing server(SMTP): smtp.mobile.mail.yahoo.com
Secure authentication:Login
username :your username@yahoo.co.uk
Password: your password

Yahoo!UK POP3 Servers

Account Tittle : Yahoo UK
Leave messages on Server: Checked if you only want local delete of files.i.e. if you delete on N900 it will not delete on Server.
Name : Your Name
User Name :your email@yahoo.co.uk
Password:you know what to do
email address:youremail@yahoo.co.uk
incoming server*POP3 : pop.mail.yahoo.co.uk
Secure authentication:checked (greyed)
Secure connection:SSL(POP3S) -greyed out
Port :995
Outgoing Server (smtp):smtp.mail.yahoo.co.uk
Secure authentication :Login
User name : youremailaddress@yahoo.co.uk
Password:you know what to do
Secure connection: SSL
Port :465
use connection specific SMTP Servers: unchecked

Feedback and comments welcome.
Mike C