Offering free e-mail services
Which is the best method to offer free e-mail services to a community's members?
For example, I am developing a project to build an Internet portal for my former high school, and I want every member of the institution -including five thousand students- to have their own e-mail account under the site's domain, and be able to create them on demand from the site's pages.
Which is the best method to achieve that in a PHP based system?
What should I requesting to the hosting company?