Re-enable DKIM-checks (#3935)

Re-enable keychange-denying when the From address is wrong

Reverts #3728
Closes #3735
Reopens #3700

src/decrypt.rs

@@ -99,8 +99,7 @@ pub(crate) async fn prepare_decryption(
         from,
         autocrypt_header.as_ref(),
         message_time,
-        // Disallowing keychanges is disabled for now:
-        true, // dkim_results.allow_keychange,
+        dkim_results.allow_keychange,
     )
     .await?;