You can verify given Emai address is available on the website using Endpoint /V1/customers/isEmailAvailable.
If You have provided websiteId field in the POST Body, search the email id in the given website scope.
If an email exists on the given website, the response will be false because the same email id already exists on the given website else the response will be true.
"websiteId": "2" //OPTIONAL FIELD
If the given email id is available in the website id 2, Output will be false otherwise true.