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

So when When you hit send, your ESP doesn't just beam your message directly into someone's inbox. It looks up the destination domain's MX records, opens an SMTP connection to that domain's receiving server, and hands the message off. Every step in that chain is routing: the process of directing a message from one server to the next until it lands at the right destination. Most of the time, it's invisible. You notice it only when something goes wrong.

Forwarding is a different operation entirely, and it happens after a message has already been delivered once. If a subscriber sets up a rule in their inbox to forward everything to a second address, that message gets re-sent as a new SMTP transmission from the forwarding server. The original message was already accepted. This second sending is what creates deliverability complications.

But The problem is authentication. When your ESP sends a message, SPF ties the sending IP to your domain, and DKIM signs the content with your private key. When that message gets forwarded, it goes out from the forwarding server's IP, which probably isn't on your SPF record. SPF breaks. DKIM usually survives because the signature is still in the headers and the body hasn't been modified, but some forwarding setups do modify messages (adding footers, for example), which breaks DKIM too. The result is a message that arrives with broken authentication and a higher chance of landing in spam.

ARC (Authenticated Received Chain) exists to handle exactly this. It's a set of headers that preserve the original authentication state before the forward happened, so the receiving server knows the message was legitimate when it was first delivered. Gmail and Microsoft Outlook both support ARC evaluation, so a well-authenticated original message has a better shot at surviving the forward. If you're seeing unexplained deliverability issues from a specific subscriber segment, it's worth checking whether those addresses are known forwarding setups rather than final inboxes.

To dig deeper into the signpost system that makes routing work, MX records explain how every sending server knows where to deliver. For the authentication side, SPF fundamentals will help you understand exactly what breaks when a message gets forwarded and what you can do about it.