One of the reasons that Yahoo mail doesn't work on WiFi:
Many ISP refuse to allow SMTP (port 25) connections for out-bound e-mail unless it goes through their SMTP servers. When you are connected via WiFi you are using ISP's Network and therefore follow their rules.
I'm still unable to connect to server. I'm with att. How do I know what att's settings are? I put in all the yahoo stuff but changed yahoo to att. It still says unable to connect. My gmail is fine. I'm using manual also.