ARC Support To Prevent Forwarded Emails Being Blocked

With the major ESPs increasingly insisting on accepting only Authenticated emails (senders having DMARC records where p=quarantine or p=none), it is no more important and timely that Zimbra provide ARC support.

Without ARC support, customers forwarding emails, or replying to a Distribution List that contains external recipients, will increasingly find such emails are rejected outright instead of merely marked as spam.

See https://support.google.com/a/answer/13198639 for background.