If you're only down for a few hours then the email will get delivered, most mail server will (normally) keep trying for up-to five days before returning it to the sender as undeliverable. My suggestion would be to use an external server for backup (ISP mail servers aren't the most reliable), I use
easyDNS to host my DNS records (it does cost money but it's relative inexpensive) and they also provide a mail backup service, if my server can't be contacted they will try to deliver for up to five days, when my server is online it will deliver the mail. You will find other service providers that have this feature if you search the web. IMO, that's a far better solution than using fetchmail.
You'll find a list of
Backup articles in the wiki. If you want an arching solution for the Open Source version then check a product called
mailArchiva - there's a paid and free version of the product.