To run an email server, today, an existing host only needs to make the
server software operational. No other change is required. For example, no
coordination with DNS administration is required.
I suppose that's still true in theory, but when's the last time you saw a
non-spam message sent to someone(_at_)[12(_dot_)34(_dot_)56(_dot_)78] or saw an MTA deliberately
configured to accept mail to dotted quad addresses? Every useful mail
server has at least an A record.
In other words, the change moves the MX from being an administrative
convenience to instead being a core requirement, ie, barrier to deployment.
Quite true. If the goal is to maximize the total amount of mail delivered
to mail servers, legit mail, spam, blowback, phishes, whatever, then
fallback to AAAA is definitely the right thing to do. But is that really
what we're trying to accomplish?
Regards,
John Levine, johnl(_at_)taugh(_dot_)com, Taughannock Networks, Trumansburg NY
"I dropped the toothpaste", said Tom, crestfallenly.