Diff for testing between 4 and 5

+ TESTING

openarc.org provides infrastructure you may use to get familar with ARC.

++ 1. Autoresponder
Send a message to echo@openarc.org. You'll receive a response instantly.seconds later. This response contain the message as received by the service. This allowes you to inspect how !OpenARC validate your sent message.

++ 2. Forwarder
Send a message to forward@openarc.org. You'll receive your message /forwarded///forwarded// back to you.you seconds later. It's not a new one. Existing DKIM signatures are expected to be still intact after forwarding.

But you may force message modification to invalidate your existing DKIM signature: Choose the text "modify" as Subject and it will be prepended with "[MODIFIED]: " on forwarding. Your DKIM signature will be no longer valid. We assume you sign the RFC5322.Subject header as advised by [https://tools.ietf.org/html/rfc6376#section-5.4 RFC 6376 - DKIM Signatures]