pub(crate) async fn handle_authres( context: &Context, mail: &ParsedMail<'_>, from: &str, message_time: i64 ) -> Result<DkimResults>
authres is short for the Authentication-Results header, defined in
https://datatracker.ietf.org/doc/html/rfc8601, which contains info
about whether DKIM and SPF passed.
To mitigate From forgery, we remember for each sending domain whether it is known to have valid DKIM. If an email from such a domain comes with invalid DKIM, we don’t allow changing the autocrypt key.