Bidirectional sync between Outlook and iPhone?
iOS is designed to sync media with iTunes while syncing mail and other information through online servers. Based on what I know, syncing your iPhone with Outlook means that iTunes reads your email account information (server names, login credentials, etc.) from Outlook, and configures your iPhone to access the same email accounts.

From this point, you can keep Outlook and iPhone "synchronized" by allowing them to continue accessing the same centralized email account(s). The email server behaves as the master copy, and both clients access it and retrieve the data.

You may need to configure your email server to allow IMAP or Exchange access in order to see the behavior you want. This will depend on your email provider.