Following up on the previous entry I have now got a exim4.conf that has survived basic testing. I have checked exim4 -bt for all major routers and found and fixed a fair number of bugs in the initial version.
Any further changes I would (like to) make would result in different behavior than the current exim v3 setup, so I am not wasting time on that until somebody from debian-admin says they want me to.
Conservative suggestions include copying the "deny local_parts = funky characters" settings from the default configuration, disabling domain-literals (or at least accepting only incoming postmaster@[local.ip] instead of only outgoing), and perhaps recipient verification.
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