It seems that Jin Shengya needs to throw the love letter into the mailbox. Where is the mailbox?

The mailbox is at the entrance of the reading room on the stairs on the fourth floor of the new building. Obviously, if you can't interact, it is very likely that your gameplay is difficult or extremely simple, so you should not interact. I also played several times, and the difficulty was different to find that the interaction was normal or above. But it wasn't a confession letter that was thrown in. Online said it was a confession letter, but it was actually a reverse tape. I talked to the teacher, got the blue card and took the anti-tape away. After I threw it away, I saw a confession letter in the prop column, and there was no reverse tape.