: Any non-standard file extension attached to an email can be a warning sign of malware . Attackers often use unusual extensions to bypass standard filters or trick users into opening malicious scripts.
During the Windows Vista and Windows 7 eras, Microsoft allowed OEMs and advanced users to download "OPK" tools to customize Windows installations. Sometimes, these tools were packaged with example configuration files or email client settings. A technician might have saved a settings profile for Outlook Express (the old Hotmail client) as hotmail.opk . hotmail.opk