License file ("MailLicense.xml" you received after purchase via email or download link) is loaded from the run folder of your application (AppDomain.CurrentDomain.BaseDirectory).
In case of MSSQL it's install path of the server e.g.:
C:\Program Files\Microsoft SQL Server\MSSQL11.SQL2012\MSSQL\Binn\
You can use LicenseHelper class to obtain the exact path, and the current status of the license:
string fileName = Limilabs.Mail.Licensing.LicenseHelper.GetLicensePath();
LicenseStatus status = Limilabs.Mail.Licensing.LicenseHelper.GetLicenseStatus();