Backup Basics for a Joomla! Web Site
From Joomla! Documentation
A couple of things to check before you can say your site and database backup are foolproof: - Try to reinstall; this may be a hassle, until you need the backup and find out it's bugged. - If you have the backup emailed to some storage mail account, try to download it: Some webmails will treat the files as spam and won't let you have them (Yahoo, laposte.net) even though they appear in the inbox as attachments. My own solution phpmybackuppro + gmail.