Today Domino accepts mail from hosts with any x.509 certificate. there is no verification.
It's difficult to only allow mails from trusted certificates.
But it would be important to have the information if the connection was coming from a connection with a validated certificate.
Domino should be able to use this information to detect if a mail is trustworthy in a more flexible way then RBLs are implemented today.
With rather a score value approach then a hard allow/deny for each criteria.
SMTP inbound certificate validation would be one positive criteria.
In DSAPI filters the certificate is available. But SMTP extension managers do currently not provide this information.
It would be good to have an event which provides the certificate for an application to verify.
The free partner solution SpamGeek could neverage this functionality.