APM session variable "ad.last.attr.mail" not present

Dear all,

F5 APM is on 12.1.2 Build 0.0.249 Final.

When I look at the session variables from a user when he has logged in from APM logon page, the particular variable "session.ad.last.attr.mail" wasn't present at all. I would like to "retrieve" the email address based on the username login authenticated against Active Directory. I have confirmed that the "Email" field of an AD user does have an email address.

What determines what session variables are populated by F5 APM? A previous customer has loads of session variables while some customer has less session variables. Does this got to do with different TMOS version?