[Exim] bouncing spam

I am looking for ways to stop Exim from retrying to send a
"bounce" message back to spammers.

I don't care if it tries once.

For example, I have bounces that get:
Connection refused
and
host lookup did not complete
and
User Unknown

My config (Exim 3.12) has:
auto_thaw = 12h (maybe I should set this to be more frequent?)
ignore_errmsg_errors_after = 12h

It appears that the "User Unknown" problem will disappear after 12 hours.

What should I do to make the other problems stop after twelve hours and
the unknown user to only try once and then don't try again at all.

(I want to instantly stop on bounces that return "User Unknown" -- this
seems like I am bouncing to an obvious spam.)

I am reading the specification about the Retry Rules.

Connection refused -- is this "refused"?
host lookup did not complete -- is this "timeout_DNS" ?
User Unknown -- is there a retry rule for this?

Can anyone share any ideas or examples?

Thanks,

  Jeremy C. Reed
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