Get the mail address from a FieldUserValue OR CreatedBy OR Author in CSOM

Code to get user Email from SharePoint ListItem - Created By

 //var Author = fieldVals["Author"];

FieldUserValue Author = (FieldUserValue)item["Author"];var user = postPageCtx.Web.EnsureUser(Author.LookupValue);
// load the data with the context             
postPageCtx.Load(user);
postPageCtx.ExecuteQuery();
var createdBy = user.Email;

Reference Links:

Get the mail address from a FieldUserValue Client Side vs Server Side

http://crawlspaceofprogramming.blogspot.com/2015/08/get-mail-address-from-fielduservalue.html?_sm_au_=iVVVTVW7sq4PQnWM

USING Powershell

https://www.sharepointdiary.com/2016/05/powershell-to-get-created-by-user-field-values-in-sharepoint.html