Error message: A copy of Postbox is already open
When starting up Postbox if you receive the following error message:
"A copy of Postbox is already open. Only one copy of Postbox can be open at a time."
... try the following:
Kill the Process
Using the Activity Manager application on macOS, or the Taskkill command on Windows, to kill the Postbox process.
Remove the Profile Lock
Locate your Postbox profile data and under <somestring.default> remove the hidden file called: .parentlock (on Mac) or parent.lock (on Windows).
On Windows, make sure you have the correct profile set within the "profiles" configuration file
If you copied a profile folder over, you may need to change your profiles file. Within your Postbox directory will be a file called "profiles.ini" which will contain the following lines:
Make sure that the Path=Profiles/xxxxx.xxxx is set to the same name as your default profiles folder.