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

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!

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.

Thank you very much!!

Really helpful for me!!