The charter states "This working group will develop a DNS-based
mechanism for storing and distributing information associated with that
authorization". If we limit the scope of the charter to the mechanism for storing the data as opposed to how it is applied, we can avoid restricting the use of data only at MTA level and not MUA.
I believe the opposite. I beleive that if we focus on only describing the configuration of the outgoing mail servers using the DNS in the
normative part of the spec EVERYTHING else is non-normative.
So we give an example of how to apply the data at the incomming MTA. We also provide an example of how to apply the data at an MUA with reasonable assumptions being made about the intermediate servers. But we do not require any particular approach here, this section is non-normative.