The best free email marketing software is Mautic. However, you will need to install it in your web server and integrate it with Amazon SES smtp which allows you to send upto 63,000 emails per month for free. The installation process requires basic in Knowledge in web development.
If you need a free version where you do not have to set up anything, then Mailchimp has a free plan of up to 2,000 email subscribers. Just create your account and you are ready to get started. Another alternative to Mailchimp is ConvertKit which offers a free plan of up to 1,000 email subscribers.