Internal Exchange integration with Google Apps

Currently in our office we work with a local Exchange server for all our internal emails. For our external emails, we download them from [login to view URL], using Exchange POP3 connector which then distribute the emails to each Exchange account. This email configuration has many advantages: -Emails that are internally go directly from one user to another. There is no delay as the email does not need to leave office. -In case there is a problem with internet we can still work internally. But it has one disadvantage: -As the Exchange is local, and under a firewall, we cannot access the emails from outside the office. We could access the main email server [login to view URL], but as it is not synchronized with Exchange, if we access we would see different things. We would like to use as mail server Google Apps Mail instead of mail.ourdomain.com. Please quote me for thinking on a solution, in which we could use Exchange Internally, Google Apps Email externally, and both synchronized. So if we do a change directly via web in Google Apps email, then the change is synchronized with the local Exchange account. Or if we do a change in Exchange, the change is uploaded and synchronized to the Google Apps email server.