The complexity we should look at is not the number of IPs, but the
number of DNS queries. The ideal method would allow admins to list
as many IPs as they want, as long as the list fits in one DNS record.
I suppose. I've been suggesting one SRV which can refer to one name
for which you do one A or AAAA lookup. You can have as many A records
as you want, all returned at once, with no further defererencing to
worry about. It's hard for me to imagine a plausible scenario in
which an SMTP client host would have so many addresses that the A or
AAAA records wouldn't fit in a UDP packet.
Even an extremely large domain, like the example I gave, could
arrange their servers so as to list them all in one record. Am I
right that the caching of this record would save a lot of DNS
Probably not. In any event, CSV is not SPF, and the idea is that you
have a CSV record per client host, not for the entire domain.