What is mail routing and how does it differ from forwarding?

Mail routing determines how messages travel from sender to recipient. It involves DNS lookups (MX records), server decisions about which path to take, and infrastructure configuration that directs traffic through specific gateways or filters. Routing happens during the delivery process.

Forwarding occurs after mail is received. A mailbox or server accepts the message, then resends it to a different address. This creates a new SMTP transaction with different envelope information, which can break authentication. The forwarding server becomes an intermediary sender.

The key difference: routing is transparent to authentication (the original sender's identity stays intact), while forwarding can strip or invalidate SPF and DKIM signatures. ARC (Authenticated Received Chain) was created to help preserve authentication through forwarding scenarios.

Routing chooses which channel the ship sails through; forwarding transfers cargo to a different ship mid-journey.