I have found that the mime creation tools in web methods do not account for the need to include the boundary attribute in the http header when posting a mime message. To get around this, I take the envelopeStream created and convert it to bytes. Take the bytes and convert to string to get the boundary info, then apply the boundary manually to the http post header content-type.
on a guide to build the mime message correctly look under delivered wm samples: 20150305113048111_000_WmSamples - sample.mime:build_MultipartMIME
hope this helps.
#webMethods-General#Integration-Server-and-ESB#webMethods