Last time I looked into it (I run a mailserver and mailman list for one of my hobby groups), S/MIME wouldn't change your "spamminess" reputation score.

DKIM, DMARC, and SPF do, though, and basically are table stakes if you want your mail (especially mailinglist messages) to go through to people at major providers.

Why are you asking? Is there a relation between spam handling and usage of S/MIME?

No, would that help? I am however using SPF and one of DKIM / DMARC (I forget which). But anyway, I can live with a mail missing here and there. It is just annoying and isn't right.