For a start this behaviour is totally untested in the wild - I've already pointed out several failure modes and there could be a whole raft of things that you and I haven't thought of that could cause issues which is why it should be optional and off by default.

In fact - I've just thought of another - if Haraka is sending mail to a smart host for delivery (e.g. via smtp_forward or default forced routing using get_mx), then this test will fail and valid mail will get nuked.