When I was working SharePoint REST API recently, I just found one interesting thing about user column regarding a query in REST API. The thing is when you use a REST to query items which contains user columns, actually you can get a bunch of properties other than just user’s ID and Title. Here is a list of these properties I copied from Stack Overflow,
Title Name EMail MobilePhone SipAddress Department JobTitle FirstName LastName WorkPhone UserName Office ID Modified Created ImnName NameWithPicture NameWithPictureAndDetails ContentTypeDisp
So here is an example,
With this call, you are going to get the user’s eamil in the JSON response.