Amavis: configure subject tag for unchecked emails
Emails that can not be checked by Amavis, e.g. because of S/MIME encryption, are passed unchecked by Amavis. Amavis does prepend UNCHECKED to the subject. This can be changed by adding the configuration option $undecipherable_subject_tag = undef; to an amavis configuration file (50-user) -- mildly obscure since it is not documented by Amavis. My suggestion is to add this option to the spam filters (frontend) or to at least add this as a commented config option in the Amavis config (backend).