Unfortunately, there is no third-party application that I am aware of that permits global forwarding of new mail notifications to an external SMTP address. So, based on the assumptions I stated, you're really faced with two options: have remote users get into the habit of checking their e-mail via OWA on a daily/regular basis, or re-evaluate the risk of forwarding e-mail externally and allow forwarding to external personal SMTP addresses per status quo.
If you end up reconsidering option two, you should know that the use of Contacts/Custom Recipients can minimize some of the threat you may be facing by giving you control over who can forward mail externally.
You can disallow mail forwarding to the Internet in Exchange System Manager. Navigate to Global Settings -> Internet Message Formats -> Default -> Properties -> Advanced and uncheck "Allow Automatic Forward" and "Allow Automatic Replies.) Then, add a new Contact/Custom Recipient for every one of your remote users' external private SMTP addresses. By doing these two things, you should substantially limit the risk.
Additionally, make sure staff termination procedures are updated to remove the Contact/Custom Recipient and disable forwarding rules for any employee upon termination.
Do you have comments on this Ask the Expert question and response? Let us know.