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Here is a WordPress plugin - Integration of Sendy with WooCommerce.
This plugin automatically adds WooCommerce Customers name and email to Sendy List after the order status is marked completed.
Here are a few ways this plugin can help you quickly build your email list, connect with your audience, and accelerate the growth of your business:
Let me know if you want any features in this plugin. or feel free to contribute on Github - https://github.com/ProThoughts/wc-sendy
I have not checked this out, but it seems a really bad idea to automatically add customers to marketing email lists without their express permission. That violates all the SPAM regulations, so I would suggest using caution with that approach.
You can ask customer permission on checkout page or add in terms and conditions page that you will be sending promotional emails or send a email to confirm for joining promotional emails. there are various ways you can get consent from customer. This is the practice normally all sites follow.