What does the built in eM Client Backup contain? Does it capture everything - mails, tasks, contacts, calendar, Rules, Local folders, preferences, themes, settings, etc?
When one restores this backup then is everything restored?
Everything except for custom themes and custom dictionaries are included in the backup.
When you restore a backup everything else is restored. However, synced folders will immediately revert to what is on the server. If you want to restore data that has been deleted from the server, you can try the restore in offline mode described here.