A postie is a person who delivers the mail — a friendly, informal word for a postman or postwoman. You hear it in British and Australian English, in speech more than in stiff writing. The postie came means the mail arrived in someone's hands, not that a box was emptied. Americans do not really use postie; they say mail carrier or mailman. The -ie ending makes the word small and warm, like goalie. This post is mail again, not after. It is kind, not rude, but it is casual: fine with a neighbour, odd in a formal letter to the post office.