How do I send a m e-mails of more than 500?

I work for a non-profit and have about 800 people who have signed up to receive our newsletter. I "inherited" a m e-mail account when I took this job, but it is no longer working. How do I set up a m e-mail account that will allow people to "subscribe" and "unsubscribe?" Does Yahoo offer this sort of service for businesses?