MailStore Home is the way to go.
MailStore ? Email Archiving Solutions
I also have the need to backup my emails (plus get protection against crashes).
I store every email received/sent onto an external hard disk using MailStore. This is free email archiving and email backup software for home users. I've been using it for over a year now, and it is great. Currently have 41,028 messages and 10,575 Mb stored, and there is no degradation in speed of search or retrieval.
The really great bit is that I backup my emails to the ExtHD just before a trip, then can take it with me when travelling and can access any e-mail from my laptop.
I normally backup all my emails to Mailstore, and then delete all but the most recent (6 months) of e-mails from my desktop PC email programme, which keeps that running smoothly. I got inspired and so am just running a backup now – it's taking about 10 minutes and processed 6,290 items (730 newly archived). For extra safety, you can even copy the Mailstore directory from the Ext HD to another USB or even to your Desktop HDD.
Their blurb says (and it is true) "This way, you will never lose emails again and can search all of your emails extremely fast. You can still reply to or forward archived emails by opening them with a single mouse click in your standard email program." I also plug my ExtHD into my desktop PC, and search and restore a single email to the desktop email. Great for when I need to reply to/quote an email that was received (or sent) a year or two ago.
It solved my problems.