User settings

No. You just need a WordPress user with Author permissions that has the same email address as you are sending from.

Hi. There’s one thing I am struggeling with is to understand the behaviour of the default WP Cron. In case there’s no traffic on the website it does not work and give me the impressione there’s something wrong with my settings. Thank you.

That is just how the wp-cron works.

See https://postieplugin.com/faq/doesnt-postie-automatically-publish-emails-running-manually-works/