Hi Hilary - we use Active Directory to import all our users and as our user's email addresses are all stored as part of their AD record, the emails just all get imported. I'm guessing by choosing to import from AD the assumption is all the data you hold about a user is stored in the AD.
We use AD to import groups rather single users. When we add a new user into the group in AD we just sync the group in Tableau.
All details are stored in AD as per Donna's reply.
Any reason why you are adding a single user rather than using groups?