Can't offer a fix, but an observation. But you might get some insight by looking at the raw email with and without. The L-Soft list server has a similar characteristic. L-Soft permits a "trailer" file which can hold the lawyer's caveat chorus or other standard notice. A single attachment converts the trailer from body-text into an attachment. I could see and explain what was happening in the raw email but, to my knowledge, they never "fixed" it to work the way I wanted it. It may be an RFC 2821 conformance issue.