Distinguish no MX from no mail
01A web address does not announce a mail service. Omitting MX records alone can still make senders try the domain’s address.
A single MX 0 . is present. The client can recognize no mail service and stop rather than use the domain’s address fallback.
Source for this drawing: RFC 7505 · Read RFC section 1 ↗
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
- MX
- Mail exchanger: a DNS record identifying a domain’s mail routing.
- A / AAAA
- DNS records containing IPv4 / IPv6 addresses.
- Implicit MX
- The fallback used when an empty MX result leads to the domain’s own address.
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
An absent mail route is different from an explicit no-service route.
- SMTP normally looks for MX records first. An empty MX result can cause implicit-MX fallback to the domain’s A or AAAA addresses; a DNS error is a different case.
- A null MX is a present MX record saying no mail exchanger exists. It avoids treating a web address as an implicit mail destination.
- Null MX permits immediate delivery failure without an SMTP listener whose only job is to refuse mail. It does not remove the domain’s web or other address records.
This comparison assumes an existing domain and an empty MX result, not NXDOMAIN or a temporary DNS failure. RFC5321§5.1 specifies different error handling for those cases. The IPv4 address 192.0.2.80 is fictional documentation data. The drawing explains routing choices, not a real DNS query or a web server’s actual network behavior. No fixed retry duration is required by null MX.
Read the RFC section ↗