You’re missing the point because you’re thinking like a techie and not like a regular user. But to answer your question, 100% with 0 effort and also 0 direct cost.
In my professional life I managed email infrastructures since 20 years ago. It only got harder.
Oh im sorry i had no idea to talk to other hackers i have to go to usernews.com obv. when this is the wrong place to think like a techie.
If you achieve 100% delivery rate you will surely share the results, right? No? Oh i guess all those services simply tell you they make it easier for you while not actually achieving that - but why check?
In my professional life I managed email infrastructures since 20 years ago. It only got harder.