https://bugs.exim.org/show_bug.cgi?id=2312
--- Comment #3 from Jeremy Harris <jgh146exb@???> ---
I think RFC 5322 requires that the From: header contain a non-empty addr-spec
(as opposed to the envelope from, where it can be empty).
Typically a bounce will have something like
From: Mail Delivery System <Mailer-Daemon@???>
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