How to export email addresses from Mail app

Looking for some help exporting email addresses from the apple mail app. I run a small business and I send out a few emails to all of my clients once or twice a year and I am looking for a more stream lined, practical way to do it. My website is on Wix and I am looking to use the email marketing function they have. I need a CSV file with all the addresses i want to import but i cant export from the mail app. The addresses are not saved in contacts. I have been just adding new email addresses to the BCC list in my mail i send out, although i can drag and drop these emails into the csv file there is a lot of them and i think there must be a better way that im just not aware of. Does anyone know a way to do it?