The PR_SUBJECT_PREFIX property contains a subject prefix that typically indicates some action on a message, such as "FW: " for forwarding.
The PR_SUBJECT_PREFIX property is recommended on all message objects.
The subject prefix consists of one or more alphanumeric characters, followed by a colon and a space (which are part of the prefix). It must not contain any nonalphanumeric characters before the colon. Absence of a prefix can be represented by an empty string or by PR_SUBJECT_PREFIX not being set.
If PR_SUBJECT_PREFIX is set explicitly, it can be of any length and use any alphanumeric characters, but it must match a substring at the beginning of the PR_SUBJECT property. If PR_SUBJECT_PREFIX is not set by the sender and must be computed, its contents are more restricted. The rule for computing the prefix is that PR_SUBJECT must begin with one, two, or three letters (alphabetic only) followed by a colon and a space. If such a substring is found at the beginning of PR_SUBJECT, it then becomes PR_SUBJECT_PREFIX (and also stays at the beginning of PR_SUBJECT). Otherwise PR_SUBJECT_PREFIX remains unset.
The subject properties are typically small strings of fewer than 256 characters, and a message store provider is not obligated to support the OLE IStream interface on them. A client should always attempt access through the IMAPIProp interface first, and resort to IStream only if MAPI_E_NOT_ENOUGH_MEMORY is returned.