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://postbox.zendesk.com/entries/69230-postbox-profile-data and under <somestring.default> remove the hidden file called: .parentlock (on Mac) or parent.lock (on Windows).

Note: Since this file is hidden, you'll have to use a Terminal Command or Application to reveal hidden files.

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:

[General]
StartWithLastProfile=1

[Profile0]
Name=default
IsRelative=1
Path=Profiles/46crnd2o.Default

Make sure that the Path=Profiles/xxxxx.xxxx is set to the same name as your default profiles folder.