feat: protect the Date header

Author: link2xt

src/mimefactory.rs

@@ -879,6 +879,12 @@ impl MimeFactory {
                 && !context.get_config_bool(Config::ProtectAutocrypt).await?
             {
                 unprotected_headers.push(header.clone());
+            } else if is_encrypted && header_name == "date" {
+                protected_headers.push(header.clone());
+                unprotected_headers.push((
+                    "Date",
+                    mail_builder::headers::HeaderType::Raw("Thu, 01 Jan 1970 00:00:00 +0000".into()),
+                ));
             } else if header_name == "from" {
                 // Unencrypted securejoin messages should _not_ include the display name:
                 if is_encrypted || !is_securejoin_message {