By accident for a short time we had a gwia visible from the internet and
configured without authentication. The gwia had a relay host configured,
relaying was disabled.

The only problem that I would have expected in that situation is that
our users would be victims of spam. I thought gwia should deny messages
to other domains than its own (those configured in Internet Addressing).
But it didn't! It accepted mails for @yahoo.com and relayed it to the
relay host. Why?

Thanks,
Mirko