the easiest is to just create a form and have it use your form mailer… it will send to your address and they just put their email into the sent/return from the form… very easily done.
You could have them click a link and it would pop up a small window with the form …complete it and send… done…
Sorry this doesn,t answer your question, I believe that what you are proposing cannot be done since… yahoo, hotmail, etc are all web based email clients, i don,t think you can call their mailer functions from your site… that almost would be hacking their system to send emails…
however if i am wrong, and you can find me an example of someone actually doing that… i can decode it and tell you how they did it… if that helps.