Rated 3 out of 5 stars

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?

This review is for a previous version of the add-on (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)