Dynamic update of DNS is a no-brainer nowadays. At the same time you
update the A record of your box which changes IP address, you also
update the "RMX" record.
This implies that a dynamic update friendly option should be provided,
taking into account the fact that a receiver may be looking at a previously
cached record.
Here the DMP style IP address mechanism looks to be most appropriate it
does not matter if the policy record is stale since that does not need to
be updated.
I think if we are going to get much further here we have to decide that
there are some things we are going to break. I cannot see any value to
an IP based authentication scheme if there is a hole in it that allows
anyone to send email from an unregistered address because thats the
way they have always done it.
Clearly people who are roaming are going to have to find a way to obtain
the imprimatur of the domain name owner, either by routing their mail
through the owner's static mail relay or by persuading the owner to support
dynamic update.
Sending email that claims to come from any domain name you choose is not
a right.