We all know the pitfalls of promising unlimited anything, which is truly impossible. However, it's still a good eye catcher, and to that end Yahoo is looking to boost users of their mail service by offering unlimited storage. They have lagged a bit behind Google for a while in terms of account storage capacity, reaching 1GB a few years ago. How much is "unlimited", though? Well, according to the article, ages ago they had the capacity for over 200GB per account:

"I remember getting in a room to plan our RocketMail launch over a decade ago and worrying that our original plan of a 2MB quota wasn't enough, and that we needed to be radical and DOUBLE the storage to 4MB per account," said Yahoo! Mail's John Kremer. "It's ironic that I routinely send and receive individual mail attachments bigger than that now. Our total capacity for mail accounts back then was 200GB for all of our customers. At Yahoo!, we're now receiving more inbound mail than that every 10 minutes."
However, they have more accounts than any other provider today. That space can't be cheap, for sure. Unlimited never is, but it's quite possible they may offer more than 99% of people could ever use. At least, in today's terms. Five years from now, 2GB might be considered a bare minimum.