Would it? They even met not so long ago and then Arata chased the train, no less, on his bike. I can't really see how one mail would be such a big deal after that.
I don't think Chihaya has once in her life thought of romance, so her obsession with Arata must spring from karuta and the fact it was Arata who taught it to her. Despite knowing he dropped it for a while, she must still consider him some sort of karuta god. But who knows, maybe there are some underlying feelings she's not consciously aware of. Taichi at least is afraid there are. Arata himself gets no screentime, but based on the fact he doesn't contact Chihaya directly, it's safe to say he's not all business anymore.