You are creating a report subscription to Email, then you notice the To field is greyed out and you are unable to edit. We assume you have setup your Reporting Services SMTP successfully because the Email option was available when you created your subscription.
Locate your RSReportServer.config file on the server where Reporting Services is installed. Take a backup of this file and keep the backup in a safe place. Now edit the RSReportServer.config file and search for the tag SendEmailToUserAlias – change the value from True to False so this line should look as follows.
Once edited, save the RSReportServer.config file. There is no need to restart your Reporting Service service. Try creating your report subscription to Email again. The To field should now be unlocked and editable.