。
Mail is delivered once a day.
Literal
Mail [topic-は] 1-day 1-time deliver-[passive-される].
The frequency expression 一日一回 ('one time per day') uses 一日 as the time unit and 一回 as the frequency. No particle between them — this kind of frequency-expression is adverbial. 配達される is the passive of 配達する ('to deliver'), making an impersonal observation about delivery frequency.