How to Turn the Sending Status Window Off
In Postbox, the Sending Messages status window will appear each time you send an email. There's a hidden preferences setting that will enable you to turn this off.
- Within Preferences, click the Advanced icon, then the General tab, and finally, the Config Editor button.
- In the filter field, type: "mailnews.show_send_progress"
- Set this preference to "false." Close the Config Editor
Note: you might need to restart Postbox for this change to take effect.