There is an immediate "relationship" question here. Are we talking about
the domain "employing" those hosts or the domain being "responsible for"
those hosts? Note that an MX record lists the hosts a domain "employs"
as mail exchangers and that it has been common for a different domain
to be "responsible for" at least one of the MX hosts at some points
in the Internet's
history.
So my question one: is it valuable for a domain to be able to list the
set
of hosts it "employs" as outbound MTAs?
My question two: is it valuable for a domain to be able to list the set
of
hosts it is "responsible for" as outbound MTAs?
Interesting point. FWIW, I don't believe that a "domain" either "employs"
or "is responsible" for a set of MTA's. The "domain" isn't the
administrative entity, it's a property of the admin entity. The admin
entity is responsible, and may choose to publish details of its MTA
employment regarding particular domains. admin "has" domains, admin "has"
MTAs.
Assertions of "responsibility" are pretty much in whois (and/or dns NS ?).
Inbound "employment" is by dns MX, all (?) we need to do is fix a record
for outbound employment.
"those maintaining" make the connection (my 2p).