RCPT TO
The SMTP command that names one recipient of the message being sent; the server accepts or refuses each recipient separately.
What it means
A client sends one RCPT TO:<address> per recipient after MAIL FROM, and each gets its own reply: 250 means the server will take mail for that address, 550 5.1.1 that the mailbox does not exist, a 4xx reply that it cannot take this recipient now. RFC 5321 requires servers to accept at least 100 recipients in a transaction.
These addresses are the envelope recipients, and they alone decide where the message goes, whatever the To and Cc headers say. That is how Bcc works, and how forwarding and mailing lists reach people whose address appears nowhere in the message. Tools that check whether an address exists by starting a transaction and reading this reply are unreliable at servers that accept every recipient or check recipients only later, and Outlook.com blocks IPs that probe addresses this way, which it calls namespace mining.
Example
C: RCPT TO:<ana@example.net>
S: 250 2.1.5 OK
C: RCPT TO:<old.user@example.net>
S: 550 5.1.1 User unknownRelated terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5321, section 4.1.1.3: RECIPIENT (RCPT)rfc-editor.org
- RFC 5321, section 4.5.3.1.8: Recipients Bufferrfc-editor.org
- Outlook.com Postmaster: Troubleshootingsubstrate.office.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes