Bug#603003: closed by Don Armstrong (reply to owner@bugs.debian.org) (---NNN is a proper MIME part separator)

[積丹尼 Dan Jacobson]

No, you have just plastered the signature after the last MIME separator.
A proper MIME section would have a Content-Disposition:,
Content-Transfer-Encoding:, etc. first, then your signature, and then a
final separator.