Robert,
I have received messages forwarded from Thunderbird and had them show up.
There must be some difference. When you are constructing the message yourself, are you following all the guidelines in RFC 1049, 1341, and 1521-1524? I think there might be other involved. Essentially as I understand it, and its been a while since I read the RFC's in full, that the message/rfc822 was to indicate an attachment, or portion that complied with the older rfc822 for backwards compatibility.
Hope this helps,
Nathan C.