[http://www.smartertools.com/downloads/SmarterMail/SMConverterWizard_ME.zip]
This conversion tool will convert the following data: Domains, Users, Mailboxes, Groups, and Mailing Lists. Post offices are converted as domain aliases to the primary domain in the post office. All extra domains within the post office are also converted as domain aliases.
Ref: http://portal.smartertools.com/KB/a252/converting-imail-mailenable-mailmax-and-merak-to-smartermail.aspx
2. Install SmarterMail and disable all of the services.
3. Run the SmarterTools MailEnable to Smartermail conversion utility and allow it to migrate the domains. The reason for this is purely so that the old emails will get moved over from mailenable to smartermail so you won't loose data.
4. Archive user list and upload to new server.
5. Create domain and user account in webmail interface.
Ref: http://www.helmarchive.com/index.php_t-7736.html