The email address is not a property/attribute of the Microsoft.AD.User class in SCSM.
The email-address is stored as an object of the System.Notification.Endpoint class.
The relationship "System.UserHasPreference" is combining the AD user object with the email-address object.
This way one user could have more than one email address (user object relates to n email address objects).
Maybe this is helpful.
Regards
Andreas Baumgarten
(Please don't forget to Accept as answer if the reply is helpful)