office 365 is the way to go, 99.9% uptime SLA, all backed up, 25GB mailboxes with archive also, can access email also using iphone and blackberry, can also use outlook on pc to access email from the cloud, with a cached local copy of data.
it uses microsoft exchange, which is the email system most large companies use. you get a website, team site, 24x7 phone and web support, which is excellent.
also you could add your pst file and existing email to the office 365 system.
bear in mind that .pst files can easily get corrupted, so its best if you kept a few different copies of the .pst file.
you wouldnt have to worry about pst files if you switched over to office 365...
i have no connection with office 365, but have been using it for a number of months.