I just upgrade my iPhone to iOS 4.2. Where did all my email go?
Email is not included in the backups according to this apple support page.  This probably makes sense as you would normally have a copy on a server whereas with SMS, MMS etc you wouldn't necessarily have a copy on the server.  So the answer to where is your email gone is it gets wiped by the backup process.

To get your email back you should be able to just go into your email accounts and click the circular arrow button to refresh or if you're setup for push mail then it should sync automatically.  You can take a look in Settings > Mail, Contacts, Calendars for your mail accounts which should still be there.  If they're not syncing try turning the mail off and back on in one of the account setting screens.