A company that I work for just acquired another company that was using Google Apps for it's email. We use Microsoft Outlook Exchange. The founders of the company (who are leaving) want to maintain a few email addresses on Google Apps for them to use over the next year. Does anyone know if it's possible to have just a few Google Apps email addresses working on a domain and the rest of the domain running on Outlook Exchange?
I work for a Google Partner and can confirm that this is possible. The best option is to perform "dual delivery" for your users, that way you can move the users who are on Exchange across later without losing any mail or having to waste any time migrating their inboxes.
You will need to decide if you want Gmail to be the primary and forward all emails to Exchange or if you want to have Exchange as the primary and forward all to Gmail.
Details on this can be found at this link: http://support.google.com/a/bin/answer.py?hl=en&answer=96855