if it is redundant there has to be something else that's eqvuivalent.
So what is that exactly?
It's the more or less obvious concept that anything between MUA via
MSA to "MO" (mail out), or anything betwen MX via MDA to MUA can be
outsourced. There is a trust relationship within both sides of this
picture (MON or MRN), and there's one "critcical hop" where unknown
strangers (last MO on the "MON" side to first MX on the "MRN" side)
talk to each other.
To get something more interesting you need "mediators" as explained
in the I-D. In an earlier I-D gateways were classified as a special
kind of MUA, I think at some point any abstraction can be too detailed
or too simplified. The MX concept ("MON" to "MRN") is IMO essential
to get the whole picture.
Maybe ADMD is a useful shorthand for "potentially outsourced agent
of some kind, or agents typically not under the same administration
as its neighbours in the mail flow figure" (because that would be a
mouthful). But ADMD is not on the same level of abstraction like
MON, MRN, or mediator (as a combo of MRN + MON).