Mark Invoices as 'Sent'

Individual invoices can be marked as 'Sent' by checking the 'Mark Invoice As Sent' checkbox next to the 'Send' button in the email screen, or by checking the 'Sent' box towards the bottom of the invoice screen.
The Sent indicator is shown in the 'Email/Review Invoices' page which is accessed via Sales/Hire > Email/Review Invoices.

All invoices where the Balance Due does not equal zero are shown in the Email/Review screen.  The 'Filters' button provides a checkbox option to Exclude Sent, which effectively lists all outstanding invoices not yet sent.