BU is a temporary file created by Pegasus Mail email client program. Pegasus Mail was developed 1989 by a single person, David Harris, who has since been the sole developer and maintainer of the program. BU files are generated during HTML-formatted email message structure analysis process.

Files with BU extension cannot be opened or edited with software other than Pegasus Mail. BU files are temporary files used by Pegasus Mail for its internal operations and are removed automatically when no longer needed. Latest version of Pegasus Mail was released in 2014 and in available for Microsoft Windows systems.