Yahoo! Mailer is a ridiculously simple OS X program that does one and only one thing - it redirects mailto: links in your web browser to bring up a new message in Yahoo! Mail, instead of requiring an external mail program (such as Apple's Mail or Eudora Mail or whatever). This way you can click on mailto: links in your web browser and have them automatically open up in Yahoo! Mail (in your default web browser). No more cutting and pasting of email addresses! Everything's good to go, and you can use the email that you want to use.
This means that to use this program, you have to set it up to be your default email reader. Normally, this is done by openning up Apple's Mail program, openning the preferences, and then setting it as the default email reader. Once you've done that, you're ready. Go back to your browser and click on an email link, it should transparently and invisibly open up a new browser window with the compose screen filled in with whoever you're sending to.
You can customize the site you redirect to if you don't want to use Yahoo! Mail.