[exim] Inserting a TLS status header?

Hi exim users,

I would like to use the TLS status of incoming delivered mail to spam
filtering. My idea was to insert a header into the mail which describes
whether the mail was delivered via smtps or not (and later potentially
more info about the certificate validity or such). Then I would create a
custom spamassassin rule that triggers on this header.

Would you have an example or advice on how I could insert such a header
by editing my exim configuration?

Thanks,

Dominik