I just use the User Profile Service, which pulls from the User Profile database in SharePoint as opposed to trying to connect directly to AD from within a form. The User Profile database should mimic what's in AD as long as the profile import schedule is set to run regularly. The attribute for email is named WorkEmail, and is extremely easy to retrieve at any given time in any form as long as you have the userName(), which can be automatically retrieved by default for the user opening the form. You can also retrieve 30+ other attributes, such as Manager, Department, PreferredName, LastName, FirstName, AccountName, etc.