You can verify given Emai address is available on the website using Endpoint /V1/customers/isEmailAvailable.
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.
Action: POST
URL: <Host_Url>/rest/default/V1/customers/isEmailAvailable
HEADER: NULL
Request Payload:
{ "customerEmail": "rakeshjesadiya@rakeshjesadiya.com", "websiteId": "2" //OPTIONAL FIELD }
Response: Boolean
If the given email id is available in the website id 2, Output will be false otherwise true.