Notes about the BU file extension:
Incorrect file associations could be the result of underlying issues within your PC system. As such it is highly recommended to scan your PC for invalid registry entries, unused processes and other unfavourable system settings to identify slowdown issues.
The BU file type is primarily associated with 'Pegasus Mail' by David Harris. Used when parsing HTML messages. If Pegasus Mail is not active, these files can be deleted; Pegasus should have deleted them.