How to intuit error: 'The letter is not from whom it claims to be.'?

Question

Foreword: p 198, Plain Words, 2014, by Ernest Gowers, revised by Rebecca Gowers, introduced me to the covert danger of 'Cannibalism by prepositions'. Then a Google search directed me to this Wordreference.com post about which I ask now.

✘ 1. The letter is not from whom it claims to be.

✓ 2. The letter is not from whom it claims to be from.

✓ 3. The letter is not from the person from whom it claims to be.

Initially, I couldn't accept that 1 was wrong. I had to work through this step-by-step proof, before I accepted that 1 was wrong, and only 2 and 3 are right. Now, I can prove that 1 is wrong; so I ask NOT about the aforesaid proof.

Instead, my problem is this: Even after seeing and working through the aforesaid proof, 1 STILL looks right to me, when it shouldn't! In other words, could someone please explain how to understand (or intuit) that 1 is wrong, WITHOUT referring to the aforesaid proof?

Answer 1

Notice that since the first part of the phrase is:

The letter is not from...

then the second part must be referring to a person (or other kind of author). So you could have:

The letter is not from John.

or

The letter is not from your mother.

Now in your example, two options for that person are toyed with:

4. ... whom it claims to be

5. ... whom it claims to be from

Those refer to different persons (or, more precisely, they do not clearly refer to the same person). So which is correct?

Well notice that 4 now sounds just plain weird (unlike your 1 which, I agree, sounded OK at first sight).
4 (The phrase "whom it claims to be" ) implies that the letter itself claims to be a person; in fact it -- the letter -- claims to be the person that wrote the letter.
That makes no sense (how can a letter be the author of itself?)

The second now makes intuitive sense, because "whom it claims to be from" is a person -- namely, the person that the letter content (or email header) asserts is the person who wrote the letter.

To make it even clearer, replace "it" in the above two phrases, with "the letter".