IMimeEntityContentId Property

string ContentId { get; set; }

Property Value

The Content-Id header is used for uniquely identifying a particular entity and uses the same syntax as the Message-Id header on MIME messages.

Setting a Content-Id allows other MimePart objects within the same multipart/related container to reference this part by its unique identifier, typically by using a "cid:" URI in an HTML-formatted message body. This can be useful, for example, when the HTML-formatted message body needs to reference image attachments.

Note
It is recommended that GenerateMessageId or GenerateMessageId(String) be used to generate a valid Content-Id value.

Reference