Before you send a receipt to an address, it is worth knowing whether the mailbox exists. You can find out at the SMTP layer without delivering anything: open a connection, say HELO, MAIL FROM, RCPT TO, and read the response. Then quit. The message is never sent.
Why RCPT TO is the interesting verb
A 250 at RCPT means the server will accept mail for that recipient. A 550 means it will not, and that is a hard signal you can act on immediately — suppress before the first send instead of after the first bounce. Greylisting shows up as 450, which tells you nothing except 'ask again later'.
What it cannot tell you
Catch-all domains accept every address, so a 250 there is meaningless. Large providers often accept everything at RCPT and reject later, or silently drop. And a mailbox that exists can still be abandoned. A positive result is weak evidence; a negative result is strong.
Etiquette that keeps you out of blocklists
Probe from a host with a proper forward-confirmed reverse DNS, honour 4xx with backoff, and never probe the same domain in parallel — receivers treat concurrent connections as abuse. Rate limit per domain, not per probe.
The detail that matters in practice
Cache the result with a short TTL and store which verb produced it. A 550 today may be a full mailbox tomorrow; a suppression based on one probe with no expiry will eventually block a real customer.
Volanea checks recipients through an SMTP probe that never sends DATA, so the check is available at signup without the delivery cost.
