Directory users email

I'm trying to use Drupal V6 to create an online directory (user list) of church members with pictures, contact info, interests, etc. Membership security access is required to view this info. This is pretty easy when I've got an email address to go with each member. But I'd like to include those in the listing that aren't on the net, for the benefit of the other members. Has anyone dealt with this issue, and how did you solve it? The problem revolves around Drupal requiring a unique email address for all users.