You can use IMAP4 with settings like this:
<parm name="SERVICENAME" value="Yahoo!" />
<parm name="SERVICETYPE" value="IMAP4" />
<parm name="INSERVER" value="imap.mail.yahoo.com" />
<parm name="OUTSERVER" value="smtp.mobile.mail.yahoo.com" />
Note this is taken from my email provxml, but you should get the idea.