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Applications DNS e-mail Internet mail determines the address of a receiving server through the DNS, first by looking for an MX record and then by looking for an A/AAAA record as a fallback. Unfortunately this means that the A/AAAA record is taken to be mail server address even when that address does not accept mail. The NULL MX RR formalizes the existing mechanism by which a domain announces that it accepts no mail, which permits significant operational efficiencies.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

This document formally defines the "NULL MX" as a simple mechanism by which a domain can indicate that it does not accept email. SMTP clients have a prescribed sequence for identifying a server that accepts email for a domain. Section 5 of covers this in detail, but in essence the SMTP client first looks up a DNS MX RR and if that is not found it falls back to looking up a DNS A or AAAA RR. Hence this overloads an email service semantic onto a DNS record with a different primary mission. If a domain has no MX records, senders will attempt to deliver mail to the hosts at the domain's A or AAAA record's addresses. However many domains do not accept email. If there is no SMTP listener at the A/AAAA address, message delivery will be attempted repeatedly for a long period, typically a week, before the sending MTA gives up. This will delay notification to the sender in the case of misdirected mail, and will consume resources at the sender. An operator could set up an SMTP listener at that address that rejects all connections (for instance with a 554 reply code as a connection-opening response) or have an MX record pointing to such a listener, to notify senders in a timely fashion. But resources (generating a bounce) will still be consumed by the sender and it requires additional services that offer little benefit to the domain. These resource usage problems are exacerbated when large volumes of email are sent using forged email addresses from a domain that does not accept email as its envelope sender, causing large numbers of bounces to be generated and to consume large amounts of resources at the senders of the bounces. This document defines a NULL MX that will cause all mail delivery attempts to a domain to fail immediately, without requiring domains to create SMTP listeners dedicated to preventing delivery attempts.

The ability to detect domains that do not accept email offers many resource savings to an SMTP server. It can choose to reject email during the SMTP conversation that presents an undeliverable RFC5321.MailFrom domain. A sending server will discover on the first sending attempt that an address is not deliverable, avoiding queuing and retries. Also, if an SMTP server accepts a message, it can be more confident that an attempt to send a Delivery Status Notification or other response will reach a recipient SMTP server.

Senders of abusive email often use return addresses with domain names that do not accept mail. These senders typically adapt quickly, and would move over to using RFC5321.MailFrom domains that have valid MX RRs. While this is true, the direct benefits to the SMTP server still apply. When an SMTP server queues a non-delivery email, the target domain will accept the email or give a definitive rejection so the queue entry will be removed promptly, thus keeping the queues short. There is also a fair amount of mail that is just misaddressed by people who mistranscribed or misunderstood an e-mail address, for example, alice@www.example.com or alice@example.org or alice@examp1e.com rather than alice@example.com. NULL MX allows a mail system to report the delivery failure when the user sends the message, rather than hours or days later.

To indicate that a domain does not accept email, it advertises a single MX RR (see , section 3.3.9) with an RDATA section consisting of preference number 0, and a dot, i.e., the DNS root, as the exchange domain, to denote that there exists no mail exchanger for a domain. (The DNS root is not a valid host name, which avoids any possibility that a NULL MX record could be confused with an ordinary MX record.) The interpretation of a NULL MX RR only applies when the domain has a single MX RR. A domain SHOULD NOT advertise multiple MX RRs including a NULL MX, but if it does, the interpretation is as described in .

The operator of an SMTP server might prefer to reject mail sent from domains that publish NULL MX, since a response or non-delivery notice will not be accepted, and legitimate mail rarely comes from domains that do not accept replies. SMTP servers that reject mail because a MAIL FROM domain has a NULL MX record SHOULD use a 550 reply code and a 5.1.2 enhanced status code. A domain that does not accept mail, as declared by NULL MX, often will also not send mail. Operators can publish SPF -all policies to make an explicit declaration that the domain is not valid in the RFC5321.MailFrom command.

SMTP mail is inherently insecure in that it is feasible for even fairly casual users to negotiate directly with SMTP servers. This specification is about eliminating one small section of SMTP insecurity. In the unlikely event that a domain legitimately sends email but does not want to receive email, SMTP servers that reject mail from domains that advertise a NULL MX risk losing email from those domains. The normal way to send mail for which a sender wants no responses remains unchanged, by using an empty RFC5321.MailFrom address. Within the DNS, a NULL MX RR is an ordinary MX record and presents no new security issues.

&RFC1035; &RFC2119; &RFC3463; &RFC5321; &RFC5598; &RFC7208;

NOTE TO RFC EDITOR: This section may be removed upon publication of this document as an RFC.

Editorial nits per Murray.

Should not publish NULL MX with other MX. Never say never. Add 5.1.2 enhanced status code. Minor editorial changes.

Editorial improvements per D. Crocker's review.

Fix typos.