MultipartRelatedXOPEncoder use MimeConstants.START_INFO_XOP ("text/xml" = TYPE_SOAP11) to set 'start-info' parameter.
But in "XML-binary Optimized Packaging" (http://www.w3.org/TR/xop10) "4.1 MIME Multipart/Related XOP Packages" is stated that start-info must contain the value of 'type' parameter from root part:
4.1 MIME Multipart/Related XOP Packages
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The root MIME part is the root part of the XOP package, MUST be a serialization of the XOP Infoset using any W3C recommendation-level version of XML (e.g., [Extensible Markup Language (XML) 1.0 (Third Edition)], [Extensible Markup Language (XML) 1.1]), and MUST be identified with a media type of "application/xop+xml" (as defined below). The "start-info" parameter of the package's media type MUST contain the content type associated with the content's XML serialization. (i.e. it will contain the same value as the "type" parameter of the root part).
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One effect of this is, that AXIS2 clients will throw an exception because they check if the SOAP Envelope namespace (SOAP11 or SOAP12) is conform to the transport Content-Type (text/xml or application/soap+xml).