I just set up my sbcglobal.net (Yahoo) on my Eris.
Go to mail and hit the menu button. You will see add new account. Under the more option on the Eris. Put in your yahoo address and password and then select set up maually.
The pop server is: pop.att.yahoo.com
The smtp server is: smtp.att.yahoo.com
Make sure for the security option you select SSL and it will put the ports in for you.
Interesting ... on my Droid I just entered my email address and it auto-configured. That said, I have 'plus' for extended access, and I know that 'free' users have had issues.
When I tried to set me up the first time automatically I got a server error over and over. Maybe it is fixed I don't know. This was a few days ago. I've seen other people having problems with yahoo mail too. Well if they are they can do it this way. Hopefully the auto set up does work now though.
I'm really not sure. Did you make sure security was set to SSL? Then the port for POP should be 995 and
the smtp port when set to ssl should be 465. Other than making sure you have a signal I really don't know.
As far as I have been told the only Yahoo acct that can be used is the PLUS acct.
The Plus acct is a POP server.
The free version is an IMAP server and Droid has a problem recognizing it.
To avoid all these configuration hasles with yahoo, I found a nifty app in the Android Market called Roid. It is an email app devoted to connecting to yahoo mail. It works even if you have Wi-Fi enabled, and it doesn't matter whether you have a free yahoo or yahoo plus, it supports both.