I do NOT disagree with what Paul has written.

Nevertheless, my opinion is that this (moving to a three-level domain,
instead of two) is a step in the wrong direction.  Having a central LDAP
directory interfaced with the mail servers handling of where the mail
should be forwarded is superior e.g., when user moves from mail system
to mail system, they do NOT have to worry about updating subscriptions
or listserv list parameterizations.

When I moved (more than a decade ago) from [log in to unmask] to
[log in to unmask] (and updated subscriptions and headers), I never looked
back.  "hostname" had changed one or twice during my tenure which would
have meant more changes including notifying correspondents.

/P