• Create an empty example.com domain in SmarterMail, configured as type External, with the Exchange server referenced by IP address. (You must create an admin account, but it does not have to be postmaster or anything else that would conflict with Exchange.) 
• Change your incoming route so that all incoming mail is sent to SmarterMail.   Verify that all mail is forwarded to Exchange and accepted (if a valid account) or discarded if invalid.
• Also in SmarterMail, create a placeholder domain called example.local, type Local.
• In Exchange, configure a route definition to send messages for example.local to smartermail.

• In SmarterMail, configure an account for user1@example.com
• Also, configure an alias that forwards user1@example.local to user1@example.local
• From an Exchange account, send a test message to user1@example.local and verify the delivery flow all the way to user1@example.com
• In Exchange, configure a contact for user1@example.local so that you can then create a Transport Rule to forward all messages for user1@example1.com to user1@example.local.  (Yes, Exchange makes this simple action rather tedious.)   Verify the mail flow.   Inform User1 that he must now log onto SmarterMail for all new messages.   
• The user can migrate his old messages on his own, or use Outlook with two defined accounts to see all his email with one interface, at least until Exchange goes away.  

Hello,

We use a great tool called MailJerry that allows you to create a batch file for migrating up to 1,000 users. It supports migrations from any platform to SmarterMail.

One thing to watch out for is the number of simultaneous migrations. You can set it to run 1, 3, 5, 10, or 20 at a time. On a quad-core server with 16GB of RAM, 10 should work, but to be safe, 5 is recommended. If you do opt for 10, keep an eye on CPU and memory usage.

What’s great is that MailJerry runs on a monthly subscription and lets you migrate as many mailboxes as you like.

Rami