If email can not be verified successfully, result will be marked “unknown”. For example, if recipient server is too slow to respond, verification result will be “unknown” (timeout) even if email address is Valid.
In plugin settings page, “Result to exclude” field will allow you to accept/reject such addresses during form submission. If you keep “unknown” box checked, it will reject such addresses with error during form submission. If you turn off the plugin, it will simply skip the email verification process and allow the form submission for any email address.
“Unknown” email address may be either valid or invalid. Recommended setting is to keep “unknown” checkbox unchecked (allow the form submission). You may find the details about the verification result in our document.