Differentiate failure to verify email address and definitive case that it is invalid

[irnc]

Currently success: true is a definitive answer that SMTP server acknowledges that it would receive email to a given address.

success: false could mean both rejection of a RCPT TO command and misconfiguration on client side, e.g. 554-Bad DNS PTR resource record. So success: false could give false negatives due to client misconfiguration.

It would be convenient to differentiate mailbox existence and middle step success flags. For example { exists: true|false|undefined, failed: 'CONNECT|EHLO|MAIL FROM|RCPT TO' } with exists set to boolean only when response is definitive.