So, let's suppose you have a really excellent forged letter of authorization saying you are authorized to read my mail. Totally convincing, anyone who sees it will have no doubt it's real.

What do you suppose you can do to give that effect?

In the US SMS case LOAs work because it's in the interests of the people who can make it happen to make it happen. The LOA is just to cover their backsides, "Why did you redirect this Michigan man's SMS messages and thereby enable his life savings to be stolen?" "Oh we had this Letter of Authorization, so we honestly believed it's what he wanted". They don't believe that, but their lawyers are confident that it's enough excuse that, at least until after somebody powerful is inconvenienced, it will hold up.

But nobody benefits from helping you give effect to your (convincing but bogus) LOA for my email. My ISP doesn't want anything to do with this work, if you're lucky they'll tell you that it's none of their business - more likely it just goes in the round file and you never hear back from them.

There is no central clearing house for email that you can give a few bucks to for my emails so long as you have an LOA to keep them on the right side of the law. Instead for email everybody involved is getting paid by me, not by you, so they've got no reason to help you at all.