Added ARC support to Inbound Messages

We have added ARC headers to all inbound email routed and forwarded by Mailgun. This change was implemented to better align with policy changes at Google and Yahoo. Also, since we can retain authentication on the forward, DMARC compliance should be retained.

We have added ARC headers to all inbound email routed and forwarded by Mailgun. This change was implemented to better align with policy changes at Google and Yahoo.

Also, since we can retain authentication on the forward, DMARC compliance should be retained.