underscore in email address rejected for user subscription.

In trying to subscribe a new use (As the site admin, I perform new author subscriptions), I'm getting the following message: "Only lowercase letters (a-z) and numbers are allowed." Since there are no uppercase letters or funky symbols, the only thing I think it could be is an underscore in the email address separating the users first and last name. Have you encountered this issue before and found a solution?