How does yahoo earn with free email service?

Yahoo and all these sites like google and msn earn all their money from advertisements. Companies pay alot to have a banner adversiting their products on these sites.

How does yahoo earn with free email service?
They have small ads on the e-mail service, next to search results and throughout the website. That is how Yahoo! earns. And they earn more when people click on the ads, so they make the ads relevant to what you are looking at.