There needs to be a 'reference' for every attachment PART in a MIME message.There is no restriction in the number of attachment references in the manifest element. The SOAP mustUnderstand attribute indicates whether a header entry is mandatory or optional for the recipient to process. If mustUnderstand="1" or "true" then this indicates that the receiver processing the Header must recognize the element. If the receiver does not recognize the element it will fail when processing the Header. Contains a reference to the MIME content part representing the attachment. It needs to always begin with "cid:" + the Content-ID within the MIME structure. It is important that the manifest description in the envelope header is that same as the the value of the appropriate "AttachmentLocation" tag in the payload!/xs:documentation