Interesting, I found out the other day that the stream coming out from the assemble stage of a send pipeline is non seekable. Kind of harsh seeing as I needed to stream through the body part. The exception message read something like “ForwardOnlyEventingReadStream does not support Seek()“. Anyhow the trick is to wrap the non-seekable stream. Check out the following SDK sample for an example, C:\Program Files\Microsoft BizTalk Server 2006\SDK\Samples\Pipelines\ArbitraryXPathPropertyHandler.