Hi,
> P.S. I saw the 'authentication-results' function in milter-headers, but not
> sure if I can use it.
That is the way to add the authentication-results header in general;
however since you use Exim which uses a legacy protocol there is
currently no possibility to add headers via Rspamd.
Your options may include:
* Adopting something like Postfix or Haraka instead
* Using Exim's own support for adding this header (if Exim is compiled
with DMARC support)
Best,
-AL.