Carry“Is it correct to say "It has (got) to be the postman" instead of "It must be the postman" when I'm making a deduction based on hearing someone knock on my front door? ”
I think it's more common to use
that. These are all possible.
That's got to be ...
That has to be ...
That must be ...
The first one sounds more informal to me, and also makes the speaker appear very sure of his deduction.
I would guess that the last one is the most often used.
CJ