Dňa 11. júla 2023 18:36:55 UTC používateľ Grant Taylor via mailop
<mailop@mailop.org> napísal:
>On 7/11/23 12:35 PM, Michael Orlitzky 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.
I was curious, as i consider A/AAAA targeting as part of SMTP,
thus i search that RFC too, but not for "fallback", i search "record"
word and i found this:
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.
Then i search "implicit", but without succes, thus i asume, that this
is only definition of behaviour when domain name has not MX
record. The behavior is to use:
MX 0 that_host
But, what is "that host"? Then i search the "host" word, and found:
Hosts are known by names (see the next section);
And in next section (domain names) is:
Only resolvable, fully-qualified domain names (FQDNs) are
permitted when domain names are used in SMTP. In other
words, names that can be resolved to MX RRs or address
(i.e., A or AAAA) RRs.
Thus when host is known by its name, then "that host" means
"that domain", and "that domain" defines email target by its
MX, A or AAAA records.
My understanding is, that A/AAAA targetting is not fallback,
it is the same mechanism as MX, except that MX can "redirect"
target to another name and A/AAAA cannot that.
Or is my research/understanding wrong?
regards
--
Slavko
https://www.slavino.sk/
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