If you insist on using email as a file management system, archive system, CRM, database, contact system, knowledge management system , etc then learn how to backup your email and practise restoring your email because it will inevitably fail no matter how reliable your email server and client are. If you don't have time, then pay somebody or do the right thing and move stuff out of email to CRM, blogs, wikis, to-do systems, etc!
email is not a knowledge management system
Email is not an archival system, file system, knowledge management system or a to-do system. If you think it's anything but a dumb temporary message store, you are "doing it wrong" :-) as the kids say. Email is where knowledge goes to die as I blogged about (AFAIK Bill French coined this phrase back in 2003)
Anything valuable in email should be gardened immediately into a blog, wiki, etc. Don't expect to keep every email and don't try; it's futile and not worthy of your attention. Instead mine the knowledge in your email and keep that!