Hi,

I finally managed to fix the problem of sendmail trying to externally relay
local mail.

Quite early I found the suggestion to put 

FEATURE(`msp', `[127.0.0.1]')dnl

into submit.mc.

I looked there and it contained already that line. So I thought the problem
must be somewhere else. However it turned out that submit.mc isn't used and
that netbsd-msp.mc is used instead. That file reads

FEATURE(`msp', `[localhost.]')dnl

After I had changed that file as suggested above, generated a new submit.cf
from it and restarted sendmail it's working again, even with the still
broken DNS config of the ISP regarding localhost.

I've deleted the bounces in clientmqueue and all seems to be well again.

So I'd suggest two things to be commited to the source:
* Change src/gnu/usr.sbin/sendmail/cf/cf/netbsd-msp.mc to use 127.0.0.1 as
  shown above.

* Remove src/gnu/dist/sendmail/cf/cf/submit.mc and (or at least) the logic
  that installs it into /usr/share/sendmail/cf to avoid future confusion
  about its relevance.

Bye, Chris