S/Mime issue on Mac's?

I'm surprised that setting is enabled in the default profile, I'm sure it's set to Ignore by default so it looks like someone has modified one of the default profiles (not good form, a custom is always preferred). Regardless (and if it's not too late) if you are sure that you have no need to request a client certificate (and verify it's identity) then I'd say it's safe to change the setting to Ignore. In this case it is the F5 that's to blame here but only because a default profile setting has been changed.

BTW I'd suggest you play it safe and only set it to Ignore on the custom profile. Until you better understand things it's safer to leave the default one as is.