How to only include posts that have been published since the last send out?

I would like our Automated newsletters to only include posts that have been published AFTER the last email was sent.
This number of posts can vary.

This is to avoid the subscribers receiving repeats of older posts.

For example today, in the Automated email configuration, in ‘New Posts’ there is one Post Showing.

But if I set the Theme Options ‘Max posts’ to anything more than 1 it will include that number of posts even if they have been sent before.

Is this this possible via settings or would it need to be configured in the theme template?

Thanks