This error means that the .NET version you have configured for the web mail site producing the error has version 1.1 of the .NET framework running this needs to be changed to version 2.
In older versions pre V3.5 of MailEnable this problem is reversed. MailEnable web mail and web administration may raise exceptions if versions of the .NET Framework other than v1.1 are configured as the script processor for ASP.NET.
ASP.NET changes between versions and in includes additional security validation routines that may cause ASP.NET specific applications to fail.
Versions 3.5 and later
If the above does not help then you may have to change this manually within the properties of the web mail site.
Version Pre 3.5 using ASP.NET
For the pre 3.5 versions it is possible to workaround this issue by modifying the web.config file associated with NetWebMail and/or NetWebAdmin.
These files are located as follows:
C:\Program Files\Mail Enable\bin\NETWebMail\web.config
C:\Program Files\Mail Enable\bin\NETWebAdmin\Web.config
Locate the following line within the "web.config" file:
Insert the following text into the above line:
Example:<pages validateRequest="false" enableEventValidation="false"/>
|Product:||MailEnable (Pro-2.X Ent-2.X)|
|Class:||PRB: Product Problem or Issue|
|Revised:||Wednesday, May 4, 2016|