What I mean is, they have to read the letters to check whether they're ones they can't read.

Fair enough. I think I cracked the case though: they probably have someone who isn't "them" read the letters though, a third party like another law firm or some contractor that offers that service specifically.

Someone has not read a book even if they read the opening paragraph, so the solution is likely far simpler.

Nope. The key sentence was at the end of the letter. At least we know one person who didn't read it. ;)