We are using v.3 of the API and running into an issue with this use case for a custom sign up form:   First we check for existence of the contact via GET Contact Collection passing the email of the potential contact (example@example.com). We receive empty response for the "contacts" and "contacts_count" response body elements (see code and screenshot)   https://api.cc.email/v3/contacts?status=all&email=example%40example.com&include_count=true   Since the previous call reports no contacts found (contact_count = 0), we assume the user doesn't exist and submit a POST Create Contact (with create_source, email_address and list_membership parameters.   { "email_address": { "address": "example@example.com", "permission_to_send": "implicit" }, "create_source": "Account", "list_memberships": [ "xxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxxxx" ] } We consistently get error response code 409, error_key = "contacts.api.conflict", error_message = "Validation failed: EmailAddress is not unique"     The contact does not appear in a search in the admin Contacts GUI. Is there a way that a deleted contact can still cause a conflict?    We have also tried POST Create Contact without the list_membership parameter, in case there was a bug with that.   We experience this problem in our codebase as well as in the API Reference testing tool as shown in the screenshots. Also, yes, we are OAuth'd into the desired account when using the API Reference testing tool. https://v3.developer.constantcontact.com/api_reference/index.html#!/Contacts/createContact       ... View more