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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.   

  1. Within Preferences, click the Advanced icon, then the General tab, and finally, the Config Editor button. 
  2. In the filter field, type:  "mailnews.show_send_progress"
  3. Set this preference to "false."  Close the Config Editor 

Note: you might need to restart Postbox for this change to take effect.