Engelstalig forum gaf het snelle en werkende anwoord:
Close Pegasus Mail and create a backup of the mail folder on your hard drive.
Next, rename the file named heirarch.pm to heirarch.pm-old. This file is used by Pegasus Mail to identify your mail folders and to display them within Pegasus Mail. The heirarch.pm file will be re-created when you restart Pegasus Mail because it will re-scan your mail folders.
If anything strange happens you will still have the backup available.