If they are new records set the Default Value for that Field.
For existing records use an Update Query

These will be new records. I don't know what table to set the default value in. The customer table does not store mailing list values. The only place I see mailing list values stored is in the CustomerXMailingListT. That is the junction table that ties the customers to the mailing list. That table only contains customers that have already been assigned to a mailing list. So when I import the new 1000 customers, they are not going to show up in the junction table because they have not been assigned a mailing list ID. That's where I'm having a problem I don't know where to or rather which table to assign the mailing list ID to. I know how to manually do it in the customer form but it's unreasonable to march through 1000 customers to assign a mailing list to. I keep thinking there has to be a way to automate this for initial import a thousand or several thousand emails.