Your idea to give 20GB of space changed the calculus for thousands of people. I know it did for me.

Maybe you can remind people what find of outside email, such as Google, Hotmail, MSN, Outlook or other that have used to receive codes to verify their Yahoo accounts, and use this excuse to remind them that you're not providing 20GB of space, that's 4 time of those offered by Microsoft and 3 times more than what Google is providing.