Disabling Domain Keys
From Qmailtoaster
Unfortunately, domain keys are broken in Toaster. It's recommended that you disable them for the time being. This will be disabled in future releases anyway, until a suitable fix can be found.
To disable domain keys:
cd /var/qmail/bin ln -sf qmail-queue.orig qmail-queue
And then restart qmail:
qmailctl restart
That's it!