Keep sender policy and empty return paths separate
04Null MX concerns receiving mail, so using such a domain as a sender can make replies and delivery-error notifications fail.
A null reverse-path is an empty SMTP envelope return address. It contains no domain to look up, and RFC7505 does not change its meaning or use.
Source for this drawing: RFC 7505 · Read RFC section 4.2 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
- Envelope sender
- The return address supplied by SMTP MAIL FROM.
- From header
- The message-level author address field, distinct from the SMTP envelope.
- DSN
- Delivery Status Notification: a report about delivery of a previous message.
- SPF
- Sender Policy Framework: a DNS-published sending-authorization policy.
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
No receiving service is not the same as an empty return path.
- Mail systems SHOULD NOT publish null MX for domains used in RFC5321.MailFrom or RFC5322.From addresses. Doing so risks rejection by receiving systems.
- If an SMTP server chooses to reject mail because its envelope-sender or From-header domain has null MX, it SHOULD use 550 with 5.7.27. The policy choice is not universal mandatory rejection.
- A domain that sends no mail can publish SPF -all to declare no authorized senders. Null MX is not a substitute for that sending policy.
- Null MX does not replace or change SMTP’s null reverse-path. MAIL FROM:<> is an empty envelope return path used by notification messages, not a DNS MX record.
RFC5321.MailFrom is an envelope address; RFC5322.From is a message header address. The diagram’s sender-policy error is a separate conditional example, not an assertion that every receiver applies it or that header From is available at MAIL time. SPF is a separate policy mechanism with its own evaluation rules. Supporting RFC5321§4.5.5 describes messages with a null reverse-path.
Read the RFC section ↗