If your Email string is null, then ToSting()ing it will throw an exception. You should remove that method call. It's probably you're main problem.
What is the error?
Try debugging like this
if (String.IsNullOrWhiteSpace(username))
throw new ArgumentException("username is null or whitespace");
MembershipUser mu = Membership.GetUser(username);
if (mu == null)
throw new ArgumentNullException("Couldn't get a membership user for that username");
string userEmail = mu.Email; // email is already a string, do not invoke Tostring() on it
if (!string.IsNullOrEmpty(userEmail))
{
//do something
}