I want a managed mail server solution that never goes down and costs very little to provision new accounts. Is there something out there that offers essentially unlimited email accounts for any number of domains, is vendor managed and NEVER goes down?

Basically I want a Dedicated Server that I would use exclusively for email. I would like to find that solution that has already engineered a great package that incorporates the SPAM filters, virus protection, web mail, etc. where I just rent the server per month. Limitations on email is just based upon the disk space I order up on the Server and bandwidth; i.e. not a fee per email account provisioned.
Do you think Greenwebpage.com servers are ok for this? Any experts' view points?
Thanks.