> On Sep 17, 2020, at 9:30 PM, @lbutlr <[email protected]> wrote: > > This may have changed, but I doubt it. If you do not have MX records > there are definitely mail servers out there that will not send mail > to you. Exchange for one at least used to refuse to deliver mail without > an MX record. I don't know if this is still the case as I am thankfully > at least 5 years from having to deal with anyone on Exchange server.
RFC 5321 was published 2008: https://tools.ietf.org/html/rfc5321#section-5.1 The lookup first attempts to locate an MX record associated with the name. If a CNAME record is found, the resulting name is processed as if it were the initial name. If a non-existent domain error is returned, this situation MUST be reported as an error. If a temporary error is returned, the message MUST be queued and retried later (see Section 4.5.4.1). If an empty list of MXs is returned, the address is treated as if it was associated with an implicit MX RR, with a preference of 0, pointing to that host. If MX records are present, but none of them are usable, or the implicit MX is unusable, this situation MUST be reported as an error. But even prior to that: https://tools.ietf.org/html/rfc2821#section-5 Once an SMTP client lexically identifies a domain to which mail will be delivered for processing (as described in sections 3.6 and 3.7), a DNS lookup MUST be performed to resolve the domain name [22]. The names are expected to be fully-qualified domain names (FQDNs): mechanisms for inferring FQDNs from partial names or local aliases are outside of this specification and, due to a history of problems, are generally discouraged. The lookup first attempts to locate an MX record associated with the name. If a CNAME record is found instead, the resulting name is processed as if it were the initial name. If no MX records are found, but an A RR is found, the A RR is treated as if it was associated with an implicit MX RR, with a preference of 0, pointing to that host. If one or more MX RRs are found for a given name, SMTP systems MUST NOT utilize any A RRs associated with that name unless they are located using the MX RRs; the "implicit MX" rule above applies only if there are no MX records present. If MX records are present, but none of them are usable, this situation MUST be reported as an error. dates back to April 201. I would expect that 19 years is sufficient time for the news to have reached Redmond, WA. -- Viktor.
