Hightail has made an application available in Yahoo! Mail called Attach Large Files which lets you send files using YouSendIt directly from your Yahoo! Mail account. The following FAQ answers some common questions about this application. The Attach Large Files application is only available through Yahoo! Mail.
You can also tell what kind of account you have when you're sending through Attach Large Files. If the icon says you can "Select files up to 100MB" then you are a Lite user. If the icon says you can "Select files up to 2GB" then you are a Paid user.