Also, is there a function in IIS that would allow you to forward users in this fashion:
Email user Logs into Email Client (Outlook, Thunderbird, etc.) -> ->
Client becomes authenticated on IIS server -> ->
IIS Server sends the email user to the CPanel Server -> ->
CPanel Authenticates -> ->
User gets mail (that was forwarded from the IIS Server to the CPanel Server) from the CPanel Inbox -> ->
User is able to send mail from the CPanel server, and have it get transferred and transmitted through the IIS server
Something like that? Where we can send and receive mail through the CPanel server, and when people email us, will have the mail go directly to the CPanel server instead of the IIS?
cPanel has 2 different mailbox formats, mbox and maildir.
mbox is a single inbox file where all messages are stored within that.
maildir uses a new and cur folder and creates a new file for each message received.
I'm not familiar in the method Exchange handles mail but unless it has some kind of export format that can match these you might have to start over. So basically chances of getting the mail data migrated over is slim to none from my guess.
Best off to log into the cPanel system, create all the mail accounts then go ahead and change your MX records. Keep the old system running until the changes propagate and you'll be fine. You can also set your Exchange server to be a mailbag during the propagation time so all mail stores and forwards to the cPanel box.