In section 5.2.4 of the DKIM specification
(draft-allman-dkim-base-00) it says:
The "DKIM-Signature" SHOULD be inserted before any
header fields that it signs in the header field block.
To do this requires sendmail 8.13. But since it's not
a mandatory requirement, it would be nice to have a
compile option to place the inserted header field at
the end of the header block, a feature that is in the
milter API in sendmail 8.12. This would allow the
milter to be used with sendmail 8.12 if one is willing
to forego the recommendation in the spec.