Stripe, is it possible to search a customer by their email?

Stripe now allows you to filter customers by email.

https://stripe.com/docs/api#list_customers

Map<String, Object> options = new HashMap<>();
options.put("email", email);
List<Customer> customers = Customer.list(options).getData();

if (customers.size() > 0) {
    Customer customer = customers.get(0);
    ...

This is important to help ensure you don’t create duplicate customers. Because you can’t put creating a customer in Stripe and the storage of the Stripe customer ID in your system inside a single transaction you need to build in some fail safes that check to see if a particular customer exists before you create a new one. Searching customers by email is important in that regard.