Specifies contents of "Content-ID" header field.
Use this property to get/set content id of the part. Like the
Message-ID values, Content-ID values must be generated to be world-unique.
The following paragraph is taken from RFC 2045 (Freed et al.), part 7:
«The Content-ID value may be used for uniquely identifying MIME entities in several contexts, particularly for caching data referenced by the message/external-body mechanism. Although the Content-ID header is generally optional, its use is MANDATORY in implementations which generate data of the optional MIME media type "message/external-body". That is, each message/external-body entity must have a Content-ID field to permit caching of such data. »
Note, this property is read-only. Use SetContentID method to set its value.