This method applies to SmarterMail 7.x - 12.x:

Follow these steps to move a user account to another domain:

Create a new user with the same username on the new domain.

Stop the SmarterMail service.

Copy the user folder from the old domain to the new domain. By default, the user folder can be found at C:\SmarterMail\Domains\example.com\User

Restart the SmarterMail service.

Delete the user from the old domain.