5 つ星中 3つの評価を受けています

I've installed the addon in Postbox, I have inserted the API key, but doesn't work in the sense that the information don't appear in the message. Do I need to make some additonal configuration?

これは以前のバージョン (2.0.0) についてのレビューです。 

Postbox is a bit different, the MailHops data is in the sidebar (or inspector pane), not the message. Open the Inspector Pane by going to View > Show > Inspector Pane (Shift + Command + I)