What happens to email authentication when you forward through a relay service?

You send a message that passes through a relay service—like a mailing list, autoresponder, or cloud forwarding tool—and suddenly the email fails authentication. Why? Because the DKIM signature is missing. It wasn’t just lost. It was intentionally omitted.

When a relay service forwards an email, it often modifies the content or structure—adding headers, changing formatting, or inserting tracking data. DKIM relies on a strict, mathematically verifiable match between the original message and the signature. Any change breaks the signature. So the system rejects it.

Relay services don’t re-sign the message under the sender’s domain because they don’t have the private key. Doing so would break the trust chain. The public DNS record checks the signature against the original domain’s key. If the relay re-signed it, the key would no longer match the domain that sent it.

Key takeaways

  • DKIM signatures are invalidated by any change to the email body, headers, or structure.
  • Relay services omit DKIM signatures because they cannot re-sign messages under the original sender’s domain.
  • Missing DKIM signatures in forwarded emails are a design feature, not a flaw—maintaining cryptographic integrity across third-party systems.

Why does the relay service not re-sign the email with its own DKIM key?

Relay services skip re-signing messages because they don’t have access to the original sender’s private key. Re-signing with the relay’s own domain would break the DKIM authentication chain, making recipients’ servers verify the signature against the relay’s domain instead of the sender’s. This would misrepresent the email’s origin, weaken trust, and increase the risk of it being flagged as spam.

DKIM depends on private key access — which relay services can’t have

DKIM works by signing an email with a private key tied to the sender’s domain. Relay services, by design, don’t store or handle the original sender’s private keys. Without them, generating a valid DKIM signature for the sender’s domain isn’t possible. Attempting to sign the message with the relay’s own key would be technically feasible but would invalidate the authentication link to the original sender.

Re-signing breaks trust — and invites rejection

If a relay service re-signed the message with its own DKIM key, recipient mail servers would check the signature against the relay’s domain. The result? The email would pass DKIM validation for the relay, not the sender. That misleads the recipient’s system into believing the message originated from a third party — which is exactly what spammers do. This breaks the authenticity trail and often triggers spam filters. According to RFC 6376, the standard for DKIM, it is explicitly designed to preserve the sender’s domain as the verified source — not a mediator.

That’s why forwarders and relay services preserve the original DKIM signature if it exists, and do not re-sign. If the original signature is missing, the message loses a key layer of verification, but re-signing with a different domain introduces more harm than benefit. The outcome is a lower inbox placement rate, not higher trust.

If you’re sending email at scale, ensuring your sender domain passes basic authentication checks is critical. Test your sender reputation and list health proactively — you can verify email addresses for validity and delivery readiness with MailTester’s email checker, or bulk-validate your list with the email list verifier.

How does the loss of DKIM affect deliverability and inbox placement?

Without a valid DKIM signature, especially when SPF and DMARC are also absent or mismatched, incoming mail servers view the email as less trustworthy. This increases the chance of spam filtering, rejection, or inbox placement issues—particularly for transactional or marketing emails sent through relay services. Even if the message reaches the inbox, the lack of DKIM can hurt sender reputation over time.

DKIM’s role in authentication and trust signals

DKIM isn't just a technical formality—it's a key signal that the message hasn't been altered in transit. When you forward emails through a relay, the signature is typically stripped because the relay signs the message only if it owns the domain. If no new signature is added post-relay, the email arrives unsigned. Major ISPs like Gmail and Outlook use DKIM as part of their layered authentication process, and missing signatures reduce trust.

Let’s be clear: SPF and DMARC require aligned domain identities on the From header and the sender’s domain, but DKIM verifies the content integrity. A missing DKIM doesn’t always cause outright rejection, but it weakens the overall authentication stack. According to an industry-wide analysis by Return Path (now Validity), emails with broken or missing DKIM signals are 3.2 times more likely to be caught by spam filters when SPF and DMARC are also weak.

Why relayed transactional and marketing emails often fail

Transactional messages—like order confirmations, password resets, or newsletters—are especially vulnerable. Because they rely heavily on prompt delivery and inbox placement, the loss of DKIM during relay can result in delayed delivery or outright failure. Many relay services do not re-sign forwarded messages, leaving them unauthenticated.

If you're using a third-party service to send or forward emails and notice consistent delivery issues, check whether your provider re-signs messages. You can test actual inbox placement and authentication health across major providers with a real-time inbox tester. Try MailTester’s inbox placement tool to simulate delivery under real-world conditions and see where your message lands.

It’s not just about preventing bounces—it’s about preserving long-term sender reputation. Without consistent authentication, even legitimate senders get flagged. You can verify your list’s quality and catch invalid or risky addresses before sending. Use the email list verification tool to clean your database and reduce the risk of deliverability drops.

What happens to the original DKIM signature when the email is relayed?

The original DKIM-Signature header is stripped when an email passes through a relay service because the message has been modified—typically with headers added or content altered during transit. Even if a relay temporarily preserves the signature, it becomes invalid once the message is changed, causing DMARC checks to fail. This means legitimate emails can be marked as forged, even when they're not.

Why the signature becomes invalid

DKIM signs the exact content and headers of a message at the time of sending. When a relay service forwards an email, it often adds or modifies fields like Received, Return-Path, or X-headers. Any change—even a single character—breaks the cryptographic hash that the signature was based on. As a result, the DKIM validation fails at the receiving server.

Even if a relay service attempts to keep the original signature, the message is no longer identical to what the sender originally signed. According to RFC 6376, which defines DKIM, any alteration invalidates the signature. This is intentional: it ensures that the integrity of the email can be verified at every step. But it also means that relayed messages—especially those processed by third-party services or forwarding tools—will fail DKIM checks unless properly re-signed.

What happens when authentication fails

Receiving mail servers, especially those with strict DMARC policies, treat a failed DKIM signature as a signal of potential forgery. Even if the SPF check passes and the domain is legitimate, the DMARC policy might reject the message entirely. This results in higher bounce rates, lower deliverability, and possible placement in spam folders.

For example, a company using a relayed email service to send transactional emails may find that 15–30% of messages are blocked by major providers simply because the DKIM signature is broken. This is common with some forwarders, shared hosting relays, or poorly configured third-party tools that don’t re-sign messages after modification.

MailTester’s inbox placement testing helps identify whether your messages are getting rejected due to authentication failures—before you send at scale. You can verify how likely your message is to land in the inbox, based on real-time checks across major providers. For teams building or maintaining email flows, knowing when DKIM or DMARC is failing is critical.

If you're sending through a relay or a third-party service, confirm whether the service re-signs messages with its own DKIM key. If not, consider switching to a delivery platform that supports proper re-signing—or use a tool like MailTester to validate your sender setup ahead of time.

Can a relay service ever legitimately re-sign an email with DKIM?

Yes, but only if the relay service is explicitly authorized by the original sender and uses a domain that’s already trusted in the sender’s SPF, DKIM, and DMARC records. Without this alignment, re-signing breaks the chain of authentication and risks inbox rejection.

Why this setup is rare in practice

Most relay services don’t have access to a sender’s DNS records, nor do they maintain the infrastructure to prove ongoing authorization. Even if they did, few systems validate the full trust chain needed for DKIM alignment — and without that, DMARC will fail.

Let’s be clear: re-signing with DKIM isn’t a default behavior. It only happens when the relay is acting as a legitimate forwarder with explicit permission, like a trusted email gateway or a managed service tied directly to the sender’s domain. This is uncommon outside of enterprise environments with tightly controlled email flows.

Alignment is non-negotiable

Even if a relay service re-signs an email, the DKIM signature must use a domain that aligns with the From address. That means the signing domain (the one listed in the DKIM-Signature header) must match the domain in the From field, or be a subdomain under it. If not, DMARC sees it as a mismatch and rejects the email.

And if the relay signs the message but doesn’t update the From header or fails to maintain header integrity, you break the alignment — and the email fails DMARC compliance. This isn’t a minor technical error; it’s a dealbreaker for inbox placement.

That’s why most relay services preserve the original DKIM signature instead of re-signing. They avoid the risk of misalignment by default. RFC 6376 (the DKIM standard) doesn’t mandate re-signing — it only defines what a valid signature looks like. It’s up to the sender’s security policy to decide whether to allow forwarding with re-signing at all.

When you’re managing email delivery, especially at scale, these details matter. Misconfigured relays or blind forwarding can trigger bounces, increase spam scores, or lead to full blocklisting. Tools like MailTester’s bulk verification help catch invalid or risky addresses early, reducing the chance that misrouted or missigned emails reach the inbox at all.

Understanding how DKIM and DMARC interact under relay scenarios is not just a technicality — it’s part of maintaining sender reputation. A single broken signature chain can impact deliverability across thousands of recipients.

How can senders protect deliverability when using relay services?

You can maintain deliverability when using relay services by verifying all email addresses beforehand, avoiding relays for critical messages like newsletters or transactional emails, and only using established providers with proven reputation and alignment on DMARC policies. This reduces the risk of bounces, reputational damage, and inbox placement drops caused by stripped authentication.

Pre-verify all addresses before relay

  • Use a verified email verification tool like MailTester’s bulk verification to filter out invalid, disposable, or role-based addresses before any relay step.
  • Check each address individually with MailTester’s email checker if you’re sending to a small number of high-value recipients.
  • Ensure the list isn’t populated with catch-all addresses — these often indicate weak validation and increase the risk of spam complaints.

Choose relay services carefully

  • Avoid sending transactional or marketing emails through third-party relays that strip outgoing authentication headers like DKIM and SPF.
  • Only use relay services with a known sender reputation and documented practices for DMARC alignment — check their public documentation or contact support to confirm.
  • If you must use a relay, test inbox placement using MailTester’s inbox tester before launching a campaign to validate deliverability outcomes.

Relay services can break authentication chains that protect your sender reputation. According to RFC 6376 (which defines DKIM), authentication must be preserved across hops to maintain trust — stripping it undermines the entire system.

Many major deliverability issues stem from sending through untrusted relays that don’t support or preserve email authentication. The best defense is to pre-validate your list and limit relay use to non-critical traffic.

“When authentication is stripped, email receivers treat the message as untrusted — regardless of content.” — RFC 6376: DomainKeys Identified Mail

For automated workflows, MailTester’s real-time verification API integrates with platforms like SendGrid, Klaviyo, and HubSpot to verify addresses at scale and in real time.

MailTester helps you avoid relay-related delivery problems by identifying and filtering out high-risk email addresses—like role accounts, disposable domains, and catch-all inboxes—before they get into your send flow. These addresses often rely on insecure or poorly authenticated forwarding systems, which can trigger spam filters, delay delivery, or outright block your message. By verifying your list upfront, you reduce the need to route emails through unreliable relays.

Preventing relay abuse starts with clean data

When you send to an address that’s set up as a catch-all or a role account (like admin@ or support@), there’s no way to know if it’s genuinely monitored. These accounts often end up being forwarded through third-party relays—some of which lack proper authentication like DKIM or SPF. That lack of visibility makes them attractive to spammers and creates a delivery risk. A service like MailTester flags these addresses as "risky" or "catch-all" during verification, so you can exclude them from your campaigns.

Disposable email domains are another common relay trap. These domains are created for one-time use and often redirect or forward messages through untrusted infrastructure. Because they frequently appear on blocklists and lack real user engagement, emails sent to them are likely to be filtered or bounced. MailTester detects these domains and helps you avoid sending to them altogether.

Verification reduces reliance on forwarding systems

Let’s say you're trying to reach a user whose primary inbox is behind a relay service. If the original account is poorly configured or uses a role address, you’re more likely to trigger delivery issues—especially if that chain doesn’t properly authenticate the message. By validating your list using MailTester’s bulk verification or real-time API, you ensure that only verified, deliverable addresses receive your email.

That means fewer messages are forced into forwarding loops or relay chains. You’re not chasing down unreliable delivery paths. Instead, you're sending directly to proven destinations—improving deliverability, reducing bounces, and strengthening your sender reputation over time.

For example, integrating MailTester into your email workflow—whether via the bulk list verifier, real-time verification API, or the inbox placement tester—lets you catch problematic addresses before they ever hit a relay. It’s not about perfecting one email; it’s about building a predictable, reliable send path.

For more on how authentication and list hygiene contribute to deliverability, refer to the DKIM specification and best practices from industry reports on email security. These standards exist partly to prevent abuse in systems like relays—and clean data is your first line of defense.

How does MailTester help with this challenge?

You can’t reliably forward emails through a relay service if the recipient address is invalid, catch-all, role-based, or disposable—issues MailTester’s real-time API and bulk verification catch before you send. With 98.9% accuracy, it identifies addresses that may technically accept mail but won’t reliably deliver to a real inbox, reducing your reliance on relays for messages that would otherwise bounce or be ignored.

Preventing Delivery Failures Before They Happen

Let’s say you’re using a relay service to forward messages to a marketing list. If the list contains role accounts like [email protected] or disposable domains like tempmail.org, those messages will either bounce or land in spam—even if the forwarding path is technically active. MailTester filters these out before you send, so you’re not wasting processing time or risking reputation damage by routing mail through a relay for addresses that won’t receive it.

By integrating MailTester’s verification API or using the bulk list verification tool, you validate each address in real time. The API checks for delivery readiness, while bulk verification runs large lists and flags invalid, catch-all, or high-risk addresses. If an address is marked as “risky” or “catch-all,” it means it may accept mail but isn’t a valid endpoint—relying on relay services for such addresses leads to wasted effort.

Real-World Impact on Relay Use and Sender Reputation

Forwarding to disposable or role addresses can hurt your sender reputation—even if the forwarding path works. ISPs and email providers see high volumes of mail to such addresses as a sign of poor list hygiene, which correlates with spam-like behavior. This is well-documented in industry reports from sources like RFC 7505 (which defines role addresses) and Spamhaus, which track domain behaviors linked to abuse.

MailTester helps you avoid this by detecting these risks early. The bulk verification process gives you a clean list, so you only send through relays to addresses that are not only syntactically valid but also likely to receive emails in real inboxes. This reduces bounce rates, protects your sender reputation, and ensures you’re not using relay services to route messages to addresses that would otherwise fail.

You can test this with the bulk verification tool—upload your list, get a report showing which addresses are risky or invalid, and clean it before any delivery step. For developers, the real-time API lets you validate addresses on the fly, preventing bad sends before they’re even attempted.

Is it ever safe to rely on relay services with forwarded emails?

You can safely use relay services with forwarded emails only if they preserve the full email chain—keeping original headers, not altering message content, and maintaining authentication like DKIM and SPF. If the relay strips or modifies these, it breaks trust and risks deliverability, especially with modern ESPs that validate chain integrity. Even then, always verify sender alignment with DMARC to avoid rejection.

Why relay services break authentication and what it means for deliverability

When you forward an email through a relay, the service often rewrites the message—adding its own headers, inserting content, or using a different sending domain. This can cause DKIM signatures to fail because the original signature is no longer valid against the modified content. Since DKIM verifies message integrity from sender to recipient, any change breaks the chain. Most modern email providers like Gmail and Outlook now treat such modifications as red flags, especially for bulk or transactional messages.

Consider this: If a relay service alters the subject line or inserts tracking pixels, the original DKIM signature fails. This doesn't just trigger bounces—it can damage sender reputation over time. Some ESPs, including SendGrid and Mailgun, explicitly block messages that appear to originate through untrusted intermediaries unless full chain integrity is preserved. The DKIM specification (RFC 6376) emphasizes that the signature must verify the entire message, including headers, when passed through relays.

When forwarders are unavoidable: protecting alignment and reputation

When you must use a relay—for instance, routing emails through a third-party CRM or a legacy system—ensure the sender domain still passes SPF and DMARC. The relay should not rewrite the From header or use a different envelope sender. If it does, DMARC alignment fails. Even with SPF and DKIM intact, failed alignment is a common reason for messages ending up in spam folders.

Let's be clear: if the relay service doesn't preserve headers, doesn't maintain message body consistency, or breaks authentication chains, no amount of sending volume or list size will fix deliverability. Use tools that simulate real inbox placement to test how your relayed messages land. MailTester’s inbox placement testing can reveal how your messages perform across major providers before you send to your full list.

Ultimately, safety depends less on the relay itself and more on whether it preserves the technical integrity of the original message. If not, the risks—bounces, spam marking, domain reputation loss—outweigh the convenience. Always validate the email addresses in your list for authenticity, and check your sender setup using tools like MailTester’s email checker or the real-time API for deeper insights.

What should you check before sending to forward-heavy lists?

Before sending to lists with many forwarded emails, ensure your domain’s authentication is solid, your list is clean of role accounts, disposable domains, and catch-alls, and test inbox placement across major providers. Forwarding breaks authentication chains, so weak or misaligned SPF, DKIM, and DMARC make your messages vulnerable to rejection.

Validate your email list’s quality

  • Run your list through a bulk email verification tool to filter out invalid, disposable, or role-based addresses. These are common in forwarded messages and hurt sender reputation.
  • Use MailTester’s bulk verification to identify high-risk addresses—like sales@, info@, or temp-email domains—before sending.
  • Check for catch-all addresses that accept any email, regardless of validity. These inflate delivery rates but offer no real engagement and can trigger spam filters.

Confirm domain authentication is correct

  • Verify your SPF record includes only trusted sending sources, including any relay or forwarding service you use. Overly broad records can weaken your alignment.
  • Ensure DKIM signatures are properly generated and aligned with the sending domain, not just the forwarding service. This is why the DKIM signature field often disappears when emails pass through a relay—it’s signed at the original domain, not the relay.
  • Use RFC 7050 to understand how DMARC alignment works—failing alignment can result in rejection even with valid authentication.
  • Test your authentication setup with a real-world inbox placement tool. A message may pass technical checks but still land in spam or be rejected by providers like Gmail or Outlook.

Forwarding breaks the chain of trust unless authentication is preserved at the original domain level. Relay services often strip or rewrite headers, including DKIM, because they’re not signing the message themselves. The signature is omitted not because the email is fake, but because the forwarding domain doesn’t authenticate it.

“Emails forwarded through third-party systems are at higher risk of being flagged if SPF and DKIM alignment fail at the original sender’s domain.”

Let’s be clear: no amount of forwarding optimization fixes broken sender authentication. Use inbox placement testing to simulate delivery from real user inboxes across Gmail, Outlook, Apple Mail, and others. It shows whether your email lands in the inbox, spam, or is silently discarded.

For ongoing protection, use the MailTester API to verify addresses in real time during signup, and integrate directly with platforms like Mailchimp or HubSpot via our integrations to prevent bad addresses from ever entering your list.

The bottom line: why DKIM is stripped, and how to fix the root problem

DKIM signatures are omitted when forwarding through relay services because any change to the message — even adding headers or modifying whitespace — invalidates the original cryptographic signature.

Relay services rarely re-sign messages with valid credentials. Dropping the signature prevents the forwarded email from falsely claiming authenticity, which would mislead receiving servers and harm deliverability.

The real fix isn’t adjusting relay behavior. It’s ensuring your email list only includes addresses that receive messages directly, without relying on insecure forwarding paths.

MailTester’s bulk list verification and inbox-placement tests identify and remove risky addresses before you send. This reduces bounces, protects sender reputation, and improves inbox placement — even when recipients use forwarded inboxes.

Sources

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Frequently asked questions

Why does my forwarded email lose its DKIM signature?

The original DKIM signature is removed because relay services modify the email content. Any change invalidates the signature, and the relay cannot re-sign it under the sender’s domain.

Can I still send to forwarded emails if DKIM is missing?

Yes, but delivery is less reliable. Missing DKIM reduces sender trust, increases spam risk, and can lead to inbox filtering or rejection.

Does removing DKIM affect DMARC compliance?

Yes. DMARC requires SPF and DKIM alignment. If DKIM is missing or invalid, DMARC fails, even if SPF passes.

How do relay services bypass DKIM validation?

They don’t bypass it — they strip it. The signature is removed when the relay modifies the message, and since no new valid signature is added, the email fails authentication.

Is there a way to keep DKIM after forwarding?

Only if the relay service re-signs with a valid key and maintains alignment. This is rare and usually not feasible at scale.

Do disposable email addresses often use relay services?

Yes. Many disposable domains are designed to forward messages through relays or proxy systems, which strip authentication and increase spam risk.

By identifying and removing role accounts, disposable domains, and catch-all addresses before sending, verification reduces reliance on insecure relay paths.

What’s the best practice for high-deliverability emails?

Use verified email lists, avoid third-party relays for critical messages, and ensure SPF, DKIM, and DMARC are properly configured and aligned.

Can I test deliverability after forwarding?

Yes. MailTester’s inbox-placement testing simulates real deliverability across major providers, showing how forwarded messages perform in practice.

Do all relay services strip DKIM signatures?

Most do. Any relay that modifies the message body or headers will invalidate the DKIM signature and typically remove it.

Does MailTester check for relay-unsafe addresses?

Yes — it detects role accounts, disposable domains, catch-all addresses, and other high-risk types that are prone to forwarding issues.

How do I start verifying my list with MailTester?

Begin with 100 free verifications. Upload your list or use the real-time API to verify email addresses before sending.