Most of the time it's prove it by doing it off your own back.
Many moons ago (pre-smart phones) I had a high volume sending client who we noticed had an spike in opt-outs in the early stages a new sign-up process on the site.
We quickly added an optout survey, signs pointed to too much irrelevant content. Quick check on the strategy and everyone was getting everything (niche auction site).
Within a week, I'd built our first prepopulating preference centre, linked as main cta on the welcome message and put before the optout link in every email and got them to tag each campaign to category preference.
Cut churn by 80% over night, revenue obviously went up and their'sending volumes keep going up.
Within 3 months the feature was out of the box in our software and year in year out has since accounted for at least 30% of email creative revenue.
I wasn't always a deliverability guy ;-)