Problem replying comment

I’ve just installed and configured RT 3.0.9 and I’m doing some
functional test.
It seems there are problems when I try to send a mail replying to a
precedent reply mail sent
by an RT user.
When I try to do it I receive back a mail with the following subject:
Permission Denied
and with the following body: Permission Denied.

I noted this problem only if I reply from a mail account external to the
RT mail domain.

I hope somebody could help me.