I don't know the diffrence between " could't have ~" and "mightn't

English as a Second Language English Vocabulary, Grammar and Idioms
1

I saw this question in my text book.

Q. I wonder why they haven’t replied to the email I sent. Do you think they received it?

A. Maybe not. They ___________

My answer is “They couldn’t have received it.”

but “They might not have received it.” is correct.

I confuse between “could not have” and “might not have”

Please teach me the difference!

2

My take:

They might not have received it = there’s a possibility they did not receive it.
They could not have received it = there’s no way they received it.

So, only the one with “might” fits your dialog because the second speaker is uncertain whether they received it or not.

3

Thank you very much!!

Really helpful for me!!