Version 3 of SmarterMail was designed to use Version 1.1 of ASP.NET, and will return a specific Bad Data error if it is set to use any other version of ASP.NET. Resolving this issue is easily done by altering the version of .NET used by SmarterMail in the Internet Information Services (IIS) Manager.
Please be aware that this fix is only applicable for Version 3 of SmarterMail. All later versions use ASP.NET 2.0
The error in question is seen here:
Navigate to Start -> Administrative Tools -> Internet Information Services (IIS) Manager.
Once open, expand the Server Name by clicking the plus-sign next to it, then expand Web Sites, right-click the SmarterMail site, and click Properties.
Once the Properties dialogue box is open, click the ASP.NET tab.
As you can see, the site is currently set to use ASP.NET 2.0.50727 Click the drop-down arrow, and select 1.1.4322.
Once selected, click OK.
SmarterMail should now display correctly upon login.