Company e-mail (and text?) announcement list

We are interested in either consulting or building an e-mail announcement list for our company. This site/application would need to have: 1) Users with various permission levels that can be assigned to categories 2) Tiered categories such as schools and classes within the schools 3) Scalability so we can add any future expansions easily ourselves 4) Ability to write in custom html/css newsletters, or send out simple messages to individual or multiple categories If this exists in a product already, I would pay for that information as well as I have been unable to find a service that offers the customization and security we require.