When you import the emails you can set the preference (list) to associate them. Send the newsletter to that list and then, eventually, delete them. But pay attention: those are not separated list: if an address is already in your database, that address is updated with the list you set during import, so it would be better to not delete it.
Newsletter is not a tool for a single email, you should be easily send a single email with your email client and using the bcc field.