On Wed, 26 Mar 2008, John C Klensin wrote:
At the time 974 and 1123 were written, the only sort of address record
in Class=IN was an A RR, and those documents used "A RR" terminology.
The change in 2821bis essentially substituted the phrase "address
record" (or "address RR") for "A record" because (i) that seemed to be
consensus on the list at that time and (ii) there is significant,
although not universal, existing practice that is consistent with
treating IPv4 (A) and IPv6 (AAAA) RRs the same way, at least with regard
With that change to "address record", if no MX record is found, the SMTP
client is required to look for DNS names with either A or AAAA RRs,
rather than A RRs only.
There's also RFC 3974 (Jan 2005, informational) which recommends treating
AAAA like A. The most popular MTAs implement this logic.
f.anthony.n.finch <dot(_at_)dotat(_dot_)at> http://dotat.at/
HUMBER: VARIABLE 3, BECOMING SOUTHERLY 4 OR 5. MODERATE. OCCASIONAL RAIN.
MODERATE OR GOOD.
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