The PR_SUBJECT_PREFIXproperty 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_PREFIXnot being set.

If PR_SUBJECT_PREFIXis 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_SUBJECTproperty. If PR_SUBJECT_PREFIXis not set by the sender and must be computed, its contents are more restricted. The rule for computing the prefix is that PR_SUBJECTmust 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_PREFIXremains 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 IStreaminterface on them. A client should always attempt access through the IMAPIPropinterface first, and resort to IStreamonly if MAPI_E_NOT_ENOUGH_MEMORYis returned.