Comment 2 for bug 266344

You're right, what's happening is the footer is getting
attached as a separate text/plain element.

The problem is that the text/plain part of the
multipart/alternative is missing a trailing newline, so my
mail client is displaying the footer right after the last
line of the text/plain part (since my mail client displays
the text/plain part rather than the text/html).

Perhaps my mail client is at fault; it's hard to say. I
doubt it would be appropriate for MailMan to go mucking with
the original text/plain part.