its not pop3 doing the sending, its submitting to a smtp server for fowarding to its destination or direct connecting to the recipents smtp server
pop3 and imap are a way to pull your email
smtp is how mail is fowraded between servers
for email with yahoo try fetchyahoo, its perl if i remeber corecttly and i used it to migrate mail from yahoo to my mail server (just thought someone might find that handy, yahoo does indeed suck)