@DanaDebele -- The fields Shape__Length (applicable for feature records with a polyline geometry) and Shape_Area (applicable for feature records with a polygon geometry) contain geodatbase managed attribute values. Their values are computed by the geodatabase when editing the feature geometry. You should not include them in a GeoEvent Definition as there are no values you can query or calculate which will transfer to the output feature record when attempting to add / update feature records with an Add a Feature or an Update a Feature output.
Feature attributes such as Shape__Area and Shape__Length are similar to attributes like objectid, or oid and globalid -- you will see them listed in a feature service's schema when reviewing the feature service specification in the ArcGIS REST Services Directory, but they are not attribute fields you specifically add to a feature service or whose values you modify when editing feature records via a web map.
You cannot write to (or overwrite) an object identifier or global identifier value using GeoEvent Server. You also cannot write or overwrite a shape's geometrically computed area or length. You should remove these attributes from your GeoEvent Definition so they are not included in the data sent to an outbound connector. Once removed you should see their original values preserved as you use GeoEvent Server to update specific attribute field(s) you want to use to indicate things like an e-mail notification has been sent for a particular feature record.
You can review the blog article Using a partial GeoEvent Definition to update feature records for additional discussion on this. One of our Esri Support analysts, Nicole, recently added a comment to the article to detail a solution she configured which does pretty much what you want to do -- set an attribute field value to indicate a feature record has been processed (or in your case, that a notification e-mail has been sent).
