The single recipients' selection contains contact addresses that are unknown in the current project.
Please note that you have to import the email adresses to the address list* first, before you send a mailing.
Do not try and enter new email addresses in the field that is intended to restriction a mailing to certain adresses. This will only work if the addresses have been improted to the address list previously.
For a complete reference, please take a look in the manual: Send Mailings