If the LicenseHelper.GetLicenseStatus method returns LicenseStatus.Valid, subject is not changed.
The MailLicense.xml file is present in the bin directory of the application
Bin folder is not the correct place for the license file.
The license is loaded from the run folder of your application.
What is the value of:
string fileName = LicenseHelper.GetLicensePath();
Is the license file present in this location?