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!