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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?

Это отзыв для предыдущей версии этого дополнения (2.0.0). 

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)