I understand your frustration. I think we have a bit of confusion on both sides in here.
Take a look at this:
письма - if the и sound is stressed it means letters.
письма - if the а sound is stressed it means it's a letter in the genetive
So, yes, you can say почему ты не читал мне письма, которое было получено тобой вчера от Ивана?
but it's kind of uncommon to speak like that nowadays, so maxmixiv figured it was actually письма in there
So given that I can say that you can say both я не читал письма and я не читал письмо. The last one is better and sounds more natural.
If you still want to go with the first one just don't forget to stress the right syllable. The stress does matter, doesn't it?![]()