btlyahoo?
I have a basic Yahoo account, and it is working fine on the iPad. The only thing I can think of is to make sure that you have Yahoo set as the Default Account in Settings under Mail, Contacts, Calendars.
I don't, so I have to force it to use yahoo by selecting the yahoo account from my mailbox view, or changing the default. I suspect that if you have another account that is not capable of sending and it is the default (perhaps because you removed and added the account) Mail might try to send using that server instead when you are viewing the unified Inbox.
Other than that I can only supply the settings I have, and no, I can't change them either.
Server: ON
Host Name: Yahoo! SMTP Server
User Name: Optional
Password: Optional
Use SSL: ON
Authentication: cant go there
Server Port: 465
To see these settings choose your account, Select Account, and then select SMTP, then select your primary server, which should be Yahoo! SMTP Server. If there are secondary servers listed you might want to turn them off, at least for now.