。
The mail comes every other day.
Literal
Postal-delivery [topic-は] 2-days-every-other-[adverbial-に] come-[polite-ます].
~おきに ('every other, at intervals of X') is a useful time pattern: 一日おきに ('every other day'), 一時間おきに ('every other hour'). Slightly ambiguous in everyday usage — some speakers interpret it as 'at intervals of X,' meaning 'every X+1 days' — but in this context 2日おき unambiguously means 'every other day.' やって来る adds emphasis and friendliness to plain 来る.