If Courier would do that, there's a far greater possibility that a
broken SMTP client would interpret a rude socket teardown of this kind
as a temporary error, which calls for an immediate retry.
Been there, done that, FW-1 smtp proxy does this, and boy, does it
It's not just broken smtp clients. A rude socket teardown is rightly
interpreted by MTAs as a temporary error. So they try again later,
starting with the same mail that caused the problem in the first place,
causing the same error. At least with Lotus Notes (alright, that is not
an MTA) this completely halts your mailflow. With other MTAs I suspect
roughly the same behaviour.