OK, this is a total SWAG, but I remeber having the same problem at one point. I bet the accounts show up in the alias list also. I believe the problem is caused by using the firstname.lastname convention as I remember. I switched to a naming convention that doesn't use characters, just alpha numeric and I haven't experienced any problems since. Hope this helps! Also, aliases will automatically be setup based on the first name and last name values you enter (firstname.lastname and firstname_lastname) so if you set up a user with a user name of john, a firstname of John, and a lastname of Smith, the following would occur:
username john
email:
john@domain.com
John.Smith@domain.com
John_Smith@domain.com
Thanks,
John G.