Definition
Email forwarding occurs when a message sent to one address is automatically redirected to another address, either through server-level forwarding rules or client-side forwarding such as Outlook rules and Gmail filters. While forwarding is a legitimate and common practice — an estimated 5% to 15% of all marketing emails are forwarded at least once — it creates significant deliverability complications because the forwarding server is typically not authorised in the original sender's SPF record. When a mailbox provider receives a forwarded message, the sending IP address is the forwarding server, not the original sender's server. If the forwarding server is not listed in the original sender's SPF record, SPF validation fails, and the message may be rejected or classified as spam.
DKIM authentication fares somewhat better with forwarding. Unlike SPF, DKIM survives forwarding because the cryptographic signature is embedded in the email headers and travels with the message — provided the forwarding server does not modify the email body or strip headers. However, some forwarding services do modify email content for tracking, attachment scanning, or spam filtering, which breaks the DKIM signature. The ARC (Authenticated Received Chain) protocol was developed specifically to address this problem. ARC allows each server that handles a forwarded message to add its own authentication seal, preserving the original authentication results while documenting each hop. Gmail, Yahoo, and Fastmail support ARC, but adoption is not universal, meaning forwarded emails from non-ARC-supporting services can arrive at the destination with failed authentication.
Best Practices
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Set up ARC if you operate a mailing list or forwarding service: ARC preserves authentication results across forwarding hops, preventing SPF and DKIM failures that would otherwise cause legitimate forwarded messages to be rejected. If you run a group mailing list, email archiving service, or forwarding platform, implementing ARC is essential for maintaining deliverability of forwarded messages.
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Authenticate your email with all three standards — SPF, DKIM, and DMARC — to survive forwarding: Messages with strong DMARC policies (p=reject) are more likely to be rejected when SPF fails due to forwarding. Align your DMARC policy to p=none or p=quarantine if a significant portion of your legitimate email is forwarded, and work toward understanding your forwarding patterns before tightening policy.
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Identify and monitor forwarding loops in your subscriber base: A forwarding loop occurs when email is forwarded between two servers indefinitely, causing message inflation and potential blacklisting. Implement loop detection by monitoring message count per subscriber and limiting the number of hops a message can survive. Most ESPs include automatic loop detection, but verify that yours is active.
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Avoid sending transactional emails through third-party forwarding services: Password resets, order confirmations, and other time-sensitive transactional emails are frequently forwarded to secondary addresses. Because these messages often contain links that must function correctly, ensure your transactional sending infrastructure uses strict authentication and test delivery through common forwarding services such as Gmail forwarding and Yahoo Groups.
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Provide a preference centre option for subscribers who forward frequently: Subscribers who regularly forward your emails to colleagues are high-value advocates but also introduce authentication complications. Include a "share with a colleague" link in your emails that generates a forward-friendly version or direct link rather than relying on manual forwarding, preserving authentication for the original delivery.
Related Glossary Terms
BIMI
BIMI (Brand Indicators for Message Identification) is an email standard that allows brands to display their logo next to their emails in supported email clients. It requires DMARC enforcement at quarantine or reject policy.
Bounce Rate
Email bounce rate is the percentage of emails that were rejected by the receiving server before reaching the recipient. It is a key indicator of list health and data quality.
Email Deliverability
Email deliverability is the ability to land in the recipient's inbox rather than the spam folder. It depends on sender reputation, authentication, list quality, and content practices.
Double Opt-In
Double opt-in (also called confirmed opt-in) is an email signup process that requires a new subscriber to confirm their email address by clicking a verification link in a confirmation email before they are added to your mailing list.
Email Alias
Alternative email addresses or subaddressing patterns that forward to a primary mailbox, affecting subscriber identity tracking and deduplication in email marketing.
Email Authentication
Email authentication (SPF, DKIM, DMARC) is a set of technical protocols that verify an email is genuinely from the domain it claims to be sent from. It prevents spoofing, phishing, and improves deliverability.
Frequently Asked Questions
SPF works by checking whether the sending IP address is authorised by the domain's SPF record. When a message is forwarded, the forwarding server becomes the sending IP for the next delivery hop. Unless the forwarding server is listed in the original sender's SPF record — which it almost never is — SPF validation fails. The message then fails DMARC authentication if the DMARC policy is configured to reject SPF failures.
DKIM can survive forwarding if the forwarding server does not modify the email body, subject line, or any DKIM-signed headers. Many forwarding services add tracking pixels, modify links, or scan attachments, which breaks the DKIM signature. ARC (Authenticated Received Chain) was designed to preserve authentication through forwarding by allowing each server to add its own authentication seal.
ARC (Authenticated Received Chain) is an email authentication protocol that preserves the authentication results of all servers that handle a forwarded message. Each server adds an ARC seal that verifies the previous authentication results, so when the message reaches the destination mailbox provider, the entire chain of handling is visible and verifiable. Gmail, Yahoo, and Fastmail support ARC.
Not all forwarded emails have problems, but a significant percentage do. Approximately 5% to 15% of marketing emails are forwarded, and a portion of those will experience authentication failures depending on the forwarding service, the sender's authentication setup, and the recipient's mailbox provider policies. Gmail-to-Gmail forwarding generally handles authentication well; forwarding through other providers is more likely to cause issues.
You cannot reliably detect every forward, but you can identify some forwarding patterns through subscriber-level analytics — if the same email is opened from different IP addresses or devices within a short period, forwarding may be occurring. IP geolocation data that shows opens from different cities simultaneously is another signal. Some ESPs offer forwarding detection features.