向IRS ACA A2A bulkrequest发射器web服务发送请求

I am trying to send a request to the IRS's BulkRequestTransmitter service. I am constructing the message manually using C# (not WCF, I'm just creating all of the XML). The XML I've constructed is valid, and I can sign it with an X509 certificate (so I am past the point of the WS-Security error that many people struggle with for this service). I am also able to apply gzip compression to the outgoing request, and as far as I can tell, I am attaching the BulkExchangeFile properly. While working on this, I have occasionally seen errors telling me to ensure that the message is MTOM-encoded, but I have applied the appropriate headers and separators as far as I can tell, and the error message I currently receive is:I am trying to send a request to the IRS's Bulk