In your BigCommerce store's admin section, go to Marketing > Email Marketing and ensure "Allow Newsletter Subscriptions" is checked. If you sell to customers within the EU (even if your business isn't in the EU), you will need to ensure that "Tick Newsletter Checkbox" is not checked as this is not GDPR compliant (customer must opt-in not opt-out).

Anyone who checks the subscription checkbox to opt in will be subscribed to the "Accepted Marketing" list which you will find in Subscribers > Segments & Lists > Your Lists. 

If a customer unsubscribes from a Conversio email, or you unsubscribe them via the Customer section from the "Accepted Marketing" list, we will sync to your BigCommerce list and unsubscribe them there too.

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