My associates and I are building a new Open-Source sending MTA (https://github.com/KumoCorp/kumomta) and that's our interpretation of the RFC as well, so we will fall back to the A record if there is no MX record returned.
Mike -----Original Message----- From: mailop <mailop-boun...@mailop.org> On Behalf Of Michael Orlitzky via mailop Sent: Tuesday, July 11, 2023 3:45 PM To: mailop@mailop.org Subject: Re: [mailop] No MX but A: broken MTA(s) On Tue, 2023-07-11 at 13:36 -0500, Grant Taylor via mailop wrote: > > However, I don't see any mention of a-record fallback in RFC 5321. -- > I didn't chase any updates. -- I do see four occurances of "fall" in > the document, three of which are fall( )back and all three have to do > with something other than MX records vs a-records. > > As such, I'd question the veracity of current SMTP needing to support > a-record fallback. 5.1. Locating the Target Host Once an SMTP client lexically identifies a domain to which mail will be delivered for processing (as described in Sections 2.3.5 and 3.6), a DNS lookup MUST be performed to resolve the domain name... The lookup first attempts to locate an MX record associated with the name... 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. _______________________________________________ mailop mailing list mailop@mailop.org https://list.mailop.org/listinfo/mailop _______________________________________________ mailop mailing list mailop@mailop.org https://list.mailop.org/listinfo/mailop
