If you just installed SmarterMail and you’re getting redirected to the frmError.aspx page check to see if you have ColdFusion custom mappings on your site and also the application pool. When doing an upgrade you need to set the application pool to asp.net 4 and integrated mode. To remove any custom mappings, navigate to the site in IIS and open handler mappings. Remove all the mappings for ColdFusion.
This is the error you will see with CF mappings added to the site.
System.InvalidOperationException: The WebResource.axd handler must be registered in the configuration to process this request.
If you want to see full errors in SmarterMail, open the web.config and turn off custom errors.