Yes, your first RSS Campaign will send out your older posts, but it will only send posts that have been created within the past 24 hours. As for your RSS Campaign going forward, how you set your campaign will determine which posts will be sent along with your email. For daily emails, only content from the past 24 hours will be sent. For weekly emails, only content posted within the past week will be sent. Monthly campaigns, only content within the past month will be sent. If no new posts or content are posted, no email will be sent.