。
That must be the mailman.
Literal
Mail-person-[honorific-さん] [to-に] no-mistake-[plain-ない].
に違いない ('there is no mistake; it must be') is a fixed certainty-modal, stronger than だろう or かもしれない. Literally 'there is no difference/mistake to it being X.' Often used for deductive conclusions — 'it must be X because [evidence].' Common in narrative and everyday reasoning.