很多小时后，答案是我将一个内存流用于多个附件.它必须弄混了所有，并且不起作用.现在，我将foreach移到了内存流之外，并且一切正常.每个附件一个内存流.

after many hours, the answer is I was using one memory stream for multiple attachments. it must of gotten all mixed up and doesn't work. I now moved the foreach outside of the memory stream and it all works. One memory stream per attachment.