Microsoft backup file is stored in .bkf file which created by NTBackup utility, when you try to open .bkf file through NTBackup utility but it is showing an error message
Extract BKF
Adolix
InnerSystem Software Ltd
SOFTOTEC
Repair PST
SysTools BKF Repair Tool
ProLingo Software
Aha-Soft
James D. Bennett
ExpressSoft, Inc.
Softdiv Software
Puma Development
Nidesoft Inc.
Shengtao Hueng