I have not tried this with the heading change and assume that in this setup so when called by SmarterMail as remote Rspamd it would have no effect.
My understanding is that Rspamd just returns the spam score to SmarterMail and SmarterMail then decides what to do with it based on the rules.
But if you set Rspamd in front of SmarterMail in the email communication process then Rspamd is able to reject mails, put mails on the graylist, change mail concerns and only then pass the message to SmarterMail.
I currently see in the constellation with SmarterMail also no added value when Rspamd adds a subject that you can also set in SmarterMail if, for example, the spam probability is low, medium or high.
I use Rspamd simply as a replacement for Cyren, which does nothing but return the spam score.
I recommend you to make the following setting in Rspamd:
su -
nano /etc/rspamd/local.d/actions.conf
Values:
reject = 150;
add_header = 6;
greylist = 4;
Restart rspamd to take changes in effect:
systemctl restart rspamd
However, Rspamd will always return the calculated spam score to SmarterMail, no matter how you set the graylist and add_header settings mentioned above.
But if you set reject too low it will not return a spam score if it is exceeded, so I set the value very high.