We have the old mail system (IMail) and just got everything working with exchange as well. What we are trying to accomplish is phase migration from time to time. Say, in Feb, move IT dept to Exchange and move sales' dept later on March.
Exchange is hosting the same domain name as the IMail. What I did was piggy bag another domain name to exchange ,
my company 's domain is : mycomp.com
In Exchange, I added piggybag.com to the Default Recipient Policy, so the exchange will get email for mycomp.com and piggybag.com. (MX record for piggybag is added to point to the exchange)
Also I added mail forwarding for IT people in IMail so any incoming msg will be forwarded to firstname.lastname@example.org which arrives at exchange. This is all ok.
The problem is when I send a mail out to a user within the company, the exchange gets the email, but IMail doesn't. I understand why but then how can I communicate with users within the company who haven't been migrated to exchange?