What Is Email Forwarding?
How Server-Side Forwarding Works
Email forwarding is a rule that passes incoming messages from one address to another address, often on a different mail server. The original address accepts the message and then sends it on to the destination. The sender usually sees nothing unusual, while the recipient gets the message from the forwarding server.
Most forwarding is server-side, meaning the mail host applies the rule before the message reaches a mailbox. Server-side rules keep working when your computer is switched off, which client-side rules cannot do, because a mail program must be running to fetch and resend messages. Forwarding to a mailbox on the same server behaves much like an alias. Forwarding to an address on another server is where the trouble usually starts.
Why Forwarded Mail Fails Authentication
Forwarding causes problems because of how sender checks work. When a server receives mail, it checks whether the sending server is allowed to send for the domain in the envelope sender, using SPF. A forwarder is a different server, usually not listed in the original domain's SPF record, so the check fails. If the original domain publishes a strict policy, the forwarded copy can be marked as spam or rejected.
DKIM often helps, because a signature covers the message content and survives forwarding when the body is not changed. Mailing lists and footers that alter the message can break the signature, though. This is why forwarded mail is sometimes rejected even when the sender is legitimate.
The usual fix is the Sender Rewriting Scheme, or SRS. It changes the envelope sender of a forwarded message to an address on the forwarder's own domain and encodes the original sender inside that address. The forwarder's domain can then pass SPF, and bounces still reach the original sender. The visible From header does not change.
A Consultant Forwarding to a Personal Account
A consultant with a company address forwards everything to a personal account at a free provider. Clients write to the company domain, which publishes a strict SPF policy. The forwarded copies fail the check, and some arrive in the spam folder of the personal account. The consultant can reduce the problem by checking whether the company's host uses SRS, and by replying from a mailbox on the company domain so the conversation stays consistent.
Frequently asked questions
Why does my forwarded email end up in spam?
Forwarded mail often fails SPF, because the forwarding server is not authorized by the original domain. If that domain also publishes a strict policy and the message has no valid DKIM signature, the receiving server may treat it as suspicious. Forwarders that use SRS, and mail that keeps its DKIM signature intact, usually avoid most of these problems.
Does forwarding delete my messages?
It depends on the setup. Some forwarding rules keep a copy in the original mailbox, and others do not. Client-side rules that fetch and remove messages can delete them from the original mailbox. Check the option before relying on forwarding as your only record of incoming mail.