Methods of Receiving Email on BlackBerry - BlackBerryFAQ
Email is not sync'd with the blackberry. Email from your @carrier.blackberry.net address, is pushed to the blackberry as it is sent to you. It is only kept at the RIM server until it can be delivered to you. If you delete it, or lose or trash the blackberry, that email will be gone unless you have backed up the blackberry.
Your other BIS email (Yahoo or gmail, your ISP account, whatever you setup) is grabbed (my word) by RIM servers and sent to your blackberry. It still is in your Yahoo inbox or whatever you are using, so this email will not be lost if something happens to the blackberry.
When you set up email in BIS, service books are pushed to the blackberry. Folders for each account are created on the blackberry. Using yahoo as example again, you will have a yahoo folder, but yahoo email will also show in the message folder. The message folder shows everything: PIN messages, SMS, MMS, all emails, and even including phone log, if you enable that setting.