Diff for testing between 2 and 3

+ 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. 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/ back to you. It's not a new one. Ususally existingExisting DKIM signatures shouldare expected to be still intact after forwarding.

But you may force the broken DKIM signatures. Place the text "modify" somewhere in the 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 Your DKIM signature will be no longer valid.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]