The spam filtering[1] that occurs for the header information appears to have been applied to the bottom link as well - making it 404. Not sure where that is getting applied, but doing a "raw source" and checking my original inbox seems to indicate the link at the bottom of the email was sent unobscured.
The comment indicates it's trying to special case these links already, but clearly it's failing. I think maybe because it's not considering the /flat/ part of the URL. I haven't got time to test it out properly right this moment, but I think that might be it.
