You gotta change the default program that your browser uses for e-mails. I use IE and the default program it uses is Microsoft Outlook for e-mails. You need to download a Yahoo e-mailing program (Dunno if there currently is one) To change default programs on IE(7). Go to Tools > Internet Options > Programs. It should show a list of all the programs IE uses when a certain action is done (like clicking an e-mail).

The steps should be similar in other browsers as well.

If you can't find a Yahoo! e-mailing proggy, just do what I do and copy/paste. And if you can't copy/paste just remember that shit/write that shit down, lols (ghetto). But yeah, above steps should work .