Thank you for using Aspose.Email.
The printing namespace is available only in the .NET 3.5 dll of Aspose.Email. It is located in the “Program Files (x86)\Aspose\Aspose.Email for .NET\Bin” directory as shown in this screenshot. Please add a reference of this DLL to your application and try it again. Let us know if we can be of any additional help to you in this regard.