You can limit a coupon so only customers in a customer group can use it. For example, a coupon code only for people on your mailing list, or a trade-only coupon for customers tagged Trade. Customers outside the customer group can't use the coupon, even if they know the code.
Before you start
Create the customer group first. See How to create a customer group
For a mailing list coupon, create a customer group with Subscribed to Mailing List ticked.
Steps
Go to Settings > Coupons.
Click + Add Coupon, or open an existing coupon.
Set up the coupon's code, discount and other options as normal.
Under Customer Pricing Group, tick the customer group or groups that can use the coupon.
Click Save.
What happens next
Only customers in a ticked customer group can use the coupon.
WithWine checks the customer group when the coupon is applied to an order, so customers who join the customer group can use the coupon straight away, and customers who leave it can't.
On the website, customers need to be logged in so WithWine knows which customer groups they belong to.
Troubleshooting
A customer can't use the coupon: check the customer matches the customer group's rules (for example, they're subscribed to the mailing list or have the right tag), and that they're logged in.
Everyone can use the coupon: check a customer group is ticked under Customer Pricing Group and that you saved the coupon.