SPWebApplication.UseExternalUrlZoneForAlerts property
SharePoint 2013
When this is set to true and if a cross firewall url zone is configured, then it will be used in alerts' emails. If this is set to true and if a cross firewall url zone is not configured, then the Default zone url for the web application will be used in alerts' emails. Set this to false if you want legacy behaviour of having urls in alerts's emails belonging to the web application in which the alert was created.
Assembly: Microsoft.SharePoint (in Microsoft.SharePoint.dll)
Property value
Type: System.BooleanA boolean value indicating if the Cross Firewall Url zone configured for the web application must be used for urls in alerts' emails. Default is true.