feat: add option to force encryption

src/receive_imf.rs

@@ -505,6 +505,14 @@ pub(crate) async fn receive_imf_inner(
         Ok(mime_parser) => mime_parser,
     };
 
+    if !mime_parser.was_encrypted()
+        && mime_parser.get_header(HeaderDef::SecureJoin).is_none()
+        && context.get_config_bool(Config::ForceEncryption).await?
+    {
+        warn!(context, "Fetched unencrypted message, ignoring");
+        return trash().await;
+    }
+
     let rfc724_mid_orig = &mime_parser
         .get_rfc724_mid()
         .unwrap_or(rfc724_mid.to_string());