What is the RCPT TO command? (Email glossary definition)

`RCPT TO` (recipient-to) is one of the core SMTP commands. After a sender announces itself with MAIL FROM, the next step is to specify each recipient with RCPT TO. The receiving server responds with whether it is willing to accept mail for that address.

The exchange:

< RCPT TO:<john@example.com>
> 250 2.1.5 Recipient OK   ← mailbox exists

Or:

< RCPT TO:<does-not-exist@example.com>
> 550 5.1.1 Recipient address rejected: User unknown   ← mailbox does not exist

Email verification engines (including ours) lean on this exchange. The engine opens an SMTP connection to the recipient's mail server, sends RCPT TO for the address being verified, and parses the response. A 250 confirms the mailbox exists. A 550, 551, or 553 confirms it does not. The engine then disconnects (QUIT) without ever sending the actual message. See how verification works, step 5.

Two complications:

• Catch-all servers respond with 250 to every RCPT TO, regardless of whether the mailbox exists. We detect this by probing with a randomly generated address on the same domain.
• Greylisting servers reply with a temporary 450 to first attempts. Our engine retries via fallback providers. Results that still fail come back as unknown with auto-refund.