3. A method as in claim 2 , wherein obtaining the set of facts from the PIM server includes: finding a set of documents on the PIM server having a reference to the user identifier and a reference to another user identifier, and forming, as a fact of the set of facts, an aggregate fact from the set of documents; wherein generating the set of KBA questions from the set of user facts includes: generating a KBA question of the set of KBA questions from the aggregate fact.
5. A method as in claim 3 , wherein the PIM server includes an email server that accesses an email database, the email database including a set of emails, each email of the set of emails including a header and a body, the header including references to user identifiers, the body including a message; and wherein obtaining the set of facts from the PIM server includes: parsing the header and the body of an email of the set of emails to form, as a fact of the set of facts, a relationship between user identifiers, references to which are included in the header.