220.127.116.11 PidTagSubjectPrefix Property
The PidTagSubjectPrefix property ([MS-OXPROPS] section 2.1033) contains the prefix for the subject of the message. This property is set by the client but can be an empty string if there is no subject. The sum of the lengths of the PidTagNormalizedSubject property (section 18.104.22.168) and the PidTagSubjectPrefix property MUST be less than 254 characters.
More details about the processing of this property are specified in section 22.214.171.124.