What you can’t do (yet)

• You can’t post a single piece of mail from this page. The game’s way of posting first hands in all the task mail the player holds, which finishes the goals that depend on it. It then sends and removes every mailable item in the inventory. A mod cannot post one letter without that sweep, and the framework has no way to make an item mailable, so it is left out. (This is InsertMail. It calls TaskItemsManager.RemoveAllMail first, which runs TaskItemMail.OnMailed on every mail task item the player holds and completes its taskToFinish goal.)
• You can’t react to the box’s own signals. They are wiring inside the scene. (The output relays are onMailTaken, onMailInserted, onMailReceived and onMailAmount.)
• You can’t subscribe to a mailbox receiving mail here. The Mail.Received event on the economy page covers it.