[SNIPPED]
actually, assuming you now have unstable in your sources.list, those
are packages currently installed for which there are updates available
(probably unstable updates...). Be careful or you'll end up farther
than you want.
I can't answer to the specifics of sendmail and libmilter, but so long
as you have unstable in your sources.list, those kept-back packages
will appear (unless you upgrade them, of course). Probably, on the
assumption that sendmail and libmilter is sufficient for your needs,
you should change you sources.list back once those two upgrades are
done.
A