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 the Mac or the Windows Task Bar to kill the Postbox process.
Remove the Profile Lock
Locate your Postbox profile data: http://support.postbox-inc.com/entries/69230-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" 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.