A "Null MX" No Service Resource Record for Domains That Accept No Mail
RFC 7505, “A "Null MX" No Service Resource Record for Domains That Accept No Mail”, is a Proposed Standard document published in June 2015 by J. Levine, M. Delany. The canonical text is published by the RFC Editor.
Abstract
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 No Service MX RR, informally called "null MX", formalizes the existing mechanism by which a domain announces that it accepts no mail, without having to provide a mail server; this permits significant operational efficiencies.
What “Proposed Standard” means
An entry-level standards-track specification: stable, peer-reviewed and a solid basis for implementation, though it may still evolve before becoming an Internet Standard.
The canonical text of RFC 7505 is hosted at rfc-editor.org. Available in TXT,HTML.
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