Did you check the ERROR logs?
They'll need to be set to detailed, so, if they're not, change that setting and then try to re-add the new domain.
If you still don't have anything to go on, then try cycling the SmarterMail SERVICE as the server Administrator, from within Windows services.
Failing the resolution of the issue being resolved after bouncing the SmarterMail service, try uninstalling, rebooting the server (to unlock any locked files), and reinstalling SmarterMail. So long as you install using the same path as the original installation, no settings or data will be lost.
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