SMTP 521 and 556 Reply Codes
This memo defines two Simple Mail Transfer Protocol (SMTP) reply codes, 521 and 556. The 521 code was originally described in an Experimental RFC in 1995 and is in wide use, but has not previously been formally incorporated into SMTP. The 556 code was created to support the new tests and actions specified in RFC 7505. These codes are used to indicate that an Internet host does not accept incoming mail at all. This specification is not applicable when the host sometimes accepts mail but may reject particular message
What it is
Adds reply codes 521 and 556 so a host can say plainly that it does not accept mail at all, rather than accepting and discarding it.
What it solves
A domain with no mail service had no way to refuse cleanly. Senders retried for days against hosts that were never going to take delivery.
What it means to operate
This is the RFC behind the null MX record. If a domain of yours sends but never receives, publish a null MX and let senders fail fast instead of queueing against you for a week.
Bounce classifier turns an SMTP response into the action it calls for. Open bounce classifier
Normative requirements
Every sentence in this RFC carrying an RFC 2119 keyword, with the section it came from.
3 The 521 Reply Code
- SHOULDIt SHOULD be used for dummy SMTP servers whose sole purpose is to notify systems that attempt to open mail connections that the host never accepts mail.
- MAYIt MAY be used in other situations where the intent is to indicate that the host never accepts mail.
- SHOULD NOTIt SHOULD NOT be used for situations in which the server rejects mail from particular hosts or addresses or in which mail for a particular destination host is not accepted.
- MAYIf the SMTP client attempts to send additional commands other than QUIT, the server MAY either continue sending 521 reply codes or simply close the connection.