What is the BU file format?
The .bu extension is used by Pegasus Mail - a free email client developed by David Harris - for temporary and backup files created automatically during normal program operation. First released around 1990, Pegasus Mail remains actively maintained (version 4.91+) and runs on Windows only.
.bu files are generated in Pegasus Mail’s data directory (typically C:\PMAIL or a user-configured mail folder) to store transient session state and backup copies of messages being composed or recently processed. The internal format varies by context: some .bu files are plain text holding partial message drafts, while others are binary session-state records. They carry no fixed magic-byte signature and are not intended for manual editing.
These files are safe to delete when Pegasus Mail is not running - the application rebuilds what it needs on next launch. If Pegasus Mail crashes, leftover .bu files are harmless but can be cleared to recover disk space. They are closely associated with other Pegasus Mail working files such as .pmm, .pms, .pmn, and .cnm.
Note that .bu is also used as a generic backup extension by many unrelated utilities - including Unix/Linux backup tools, Oracle RMAN, and router configuration exporters - so file context matters when identifying a .bu file found outside a Pegasus Mail folder.
Security & safety
RISK: LOWPegasus Mail .bu files are internal application temp/backup files - no executable content. Generic .bu backup files carry the same risk as the source file they back up. No known malware uses .bu extension specifically.
Format details
in a nutshell- Generic backup file - Many backup utilities and scripts use .bu as a conventional backup extension, completely unrelated to Pegasus Mail.
- Oracle RMAN backup piece - Oracle Recovery Manager sometimes names backup pieces with .bu extension.
- Unix/Linux file backup copy - System administrators often create .bu copies of config files (e.g. httpd.conf.bu) before editing.
Programs that open BU files
Windows2 apps
Pegasus Mail
Free Pegasus Mail opens its own .bu files automatically as part of session recovery. Do not open manually; let Pegasus Mail manage its temp files. 
Notepad (inspect only)
Built-in Open with Notepad to inspect if the .bu is text-based (config backup, email text). Binary BU files will show garbled content. Technical details
deep spec| Platform | Windows only |
| Encoding | Binary or plain text (context-dependent - session state vs. message backup) |
| Byte order | Little-endian (Windows-native application) |
| Container | Pegasus Mail proprietary internal format |
| Magic bytes | None documented - no fixed file signature |
| Typical file size | 1 KB - several MB (email content backup) |
| Purpose | Temporary session state and email message backup during active Pegasus Mail operation |
| Safe to delete | Yes, when Pegasus Mail is closed; files are rebuilt automatically on next launch |
| Default location | Pegasus Mail data directory, e.g. C:\PMAIL or configured mail folder |
| Related Pegasus Mail files | .pmm, .pms, .pmn, .cnm |
| Extension ambiguity | Also used by Oracle RMAN backup files, Unix backup tools, and router configuration exporters |
| Released | ~1990 (Pegasus Mail 1.0) |
| Latest version | Pegasus Mail 4.91 (2022+) |
BU conversions
Community Q&A
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