custom email solution on existing site/server

We have a site that allows our users to sign up for internet packages and surf the web. We would like to add the ability for users to login through our site, connect to the internet and check, send, receive email from the same console. This should be a custom solution, that is fully integrated into our site, we would like the ability for users to add aliases, etc. to their accounts as well. If you need to add SMTP and other necessary tools to the server to make this happen please know your bid should be all encompassing. We would like to get this going as soon as possible. Thanks for your help.