With IP6 we can also very easily reach the 512 packet limit for DNS over
UDP.
Don't you think people will tend to use fewer, bigger netblocks? After
all, it's easy to fill an IPv4 /24, but hard to fill an IPv6 /48.
It may vary greatly according to policy. I can certainly imagine a
worst-case scenario where a company offering, say, DSL to customers, and
every other one elects to handle their own mail though continue to use
the same domain -- meaning you have to list each individual address in
the MARID record.
I suspect it will largely be a non-issue, though.
Ari