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Just for fun I tried it on MacOSX, but running from the shall, not qmail, and I don't see any problems there. At least, not if I pass a small input message. If I pass a large input message I get a "broken pipe", but that is to be expected (as true doesn't really read it's input).

My guess is that this is a third-party error, somehow related to how qmail sets up stdin/stdout/stderr (apparently slightly different on Free/OpenBSD and the other systems). Try finding out what stdin/stdout/stderr refer to (with fstat or something).