ContentID is a string property that represents the values used in a "Content-ID:" header field, as described in the Intenet Standards documents: 

RFC 2045 - Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies  

RFC 2046 - Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types  

ContentID acts a unique identifier the MIME entity. 

Although ContentID is generally optional, its use is mandatory in messages with the MIME ContentType 'message/external-body'. 

For messages with the ContentType 'multipart/alternative', each message part represents the same data (onlyin a different format). ContentID should have a different Content-ID value in the case where the information content of the two message parts is not identical. 

When the information content is identical, the same ContentID should be used for each of the identical message parts. 

Use ContentLocation to determine the URI for where the MIME entity can be accessed.