Detecting Multiple DKIM Signatures in an Email Header

Why Multiple DKIM Signatures in an Email Header Matter for Deliverability

You send an email. It passes SPF. It passes DKIM. But then it fails — not because the headers look wrong, but because there are two DKIM signatures where one should be. Why does this happen, and why does it matter?

Multiple DKIM signatures aren’t inherently broken. But when they appear in a single email header, they often signal a breakdown in authentication policy across a layered sending stack — a red flag to receivers that may not be expecting it. Think of it like having two separate seals on a document from different parties: the message might still be legitimate, but the inconsistency raises questions about process control.

This isn’t a common issue, but when it shows up, it’s usually a symptom, not a design choice. It points to misconfigured relay chains, overlapping third-party services, or inconsistent policy enforcement — all of which undermine sender reputation and increase risk of spam filtering or delivery rejection.

Key takeaways

• Multiple DKIM signatures in an email header often reveal misconfigurations in a layered sending stack, even if technically valid.
• Receiving systems may interpret duplicate signatures as inconsistent authentication, increasing delivery risk.
• Monitoring for multiple DKIM signatures helps catch broken or overlapping processes before they impact sender reputation.

What Causes Multiple DKIM Signatures in an Email Header?

Multiple DKIM signatures appear when an email passes through systems that each sign the message independently—common in complex email workflows involving third-party senders, content processors, or forwarding gateways. Each signature is generated at a point where the message is reshaped or relayed, even if it was already signed before. This isn’t always a flaw, but it can complicate authentication checks, especially if signatures conflict or are improperly validated.

Multiple intermediaries add signatures without coordination

Imagine sending a campaign through a marketing platform, which forwards the email to a transactional relay, then to a third-party SMTP service. Each of these systems may independently apply DKIM signing, resulting in multiple signatures. The receiving mail server validates each signature in sequence—if any fail, the email may be marked as suspicious or rejected.

Tools like MailTester’s bulk verification help identify if a list includes addresses likely to trigger delivery issues due to inconsistent or redundant signing patterns, especially if a domain sends across multiple platforms.

Content transformation triggers re-signing

Some services rewrite links, optimize images, or insert tracking pixels—actions that alter the original email content. Even minor changes invalidate an existing DKIM signature, so the system must sign the message again. If the original signature isn’t preserved or the new one isn’t aligned with standards, you get multiple signatures, one for the original and one for the modified version.

This is especially common with dynamic content platforms. The DKIM specification allows for multiple signatures as long as they’re valid and correctly structured, but receivers must still verify them individually. Misaligned or conflicting signatures can reduce reputation and delivery rates.

Misconfigured gateways or forwards add unnecessary signatures

Forwarding systems or email gateways that don’t check for existing DKIM signatures may apply new ones without review. This leads to redundant signing—especially if the original signature was valid. If the system adds a new signature using a different selector or domain, validation can fail, because the receiving server may not trust the new signing key.

For organizations managing multiple delivery paths, checking header integrity is essential. Use tools like MailTester’s inbox placement tester to simulate real delivery and spot anomalies, including overlapping or conflicting DKIM signatures, before sending to real recipients.

How to Identify Multiple DKIM Signatures in an Email Header

You can detect multiple DKIM signatures by opening an email’s raw header—use Gmail’s “Show original” to view the full source, then scan for multiple lines starting with DKIM-Signature:. Each line represents a distinct signature. Check the d= tag in each to see which domain signed it. Multiple domains aren’t inherently bad, but inconsistent or mismatched domains may indicate spoofing or poor alignment, triggering filtering systems.

Step-by-Step Process

1. Open the email in Gmail, Outlook, or another client. Click “Show original” to view the full header. This reveals the raw email structure, including all headers and signatures.
2. Scroll through the header fields and look specifically for lines starting with DKIM-Signature:. Each occurrence is a separate signature, typically spanning a few lines.
3. Examine the d= attribute in each signature. It shows the domain that signed the email. For example, d=example.com means the signing domain is example.com.
4. Compare the domains listed in d=. Multiple valid domains are normal if multiple parties are involved—like a mailing list provider and the sender. But inconsistent or unexpected domains (e.g., a signature from trusted-sender.net on a brand email from company.com) raise red flags.
5. Check for domain alignment. DKIM relies on SPF and DMARC alignment. If the signing domains don’t match the From: domain or the SPF sender, the email risks failure or delivery issues.

Why It Matters

Multiple DKIM signatures aren’t always a problem—legitimate email flows often involve intermediaries like ESPs or forwarders that add their own signature. However, inconsistent or unexpected domains can indicate spoofing, misuse of email infrastructure, or poor deliverability setup.

According to RFC 6376, the DKIM-Signature header field is defined to support multiple signatures, but each must be valid and properly aligned. Misaligned or malformed signatures are commonly flagged by receivers and can lead to inbox placement failures.

You can test the integrity of your email infrastructure through real-time inbox placement testing. Tools like MailTester’s inbox placement tester simulate how your messages land across major providers—helping catch alignment issues before mass sends.

For ongoing list hygiene and sender reputation monitoring, consider bulk testing with verified data. MailTester’s bulk verification identifies malformed or high-risk addresses—including those with misconfigured DKIM—before they impact deliverability.

Never assume multiple DKIM signatures are safe. Validate each signature’s domain and alignment. Use real-time inspection to catch issues early—before your campaigns get filtered or blocked.

When Multiple DKIM Signatures Are Acceptable (and When They’re Not)

Multiple DKIM signatures in an email header are acceptable when each signature comes from a trusted, well-maintained domain—especially in SaaS environments where third-party services or partners sign the message on behalf of the primary sender. However, problems arise when domains signing the email aren’t aligned with the message’s From: or Return-Path: domains, or when overlapping signatures from unrelated parties suggest a lack of control or consistency in the delivery chain. The absence of proper alignment with SPF or DMARC policies increases the risk of false positive spam detection or outright blocking.

When Multiple DKIM Signatures Make Sense

Let’s say you’re a SaaS platform that sends transactional emails through a partner service—like a payment processor or a notification gateway. It’s standard for that partner to add its own DKIM signature, provided it’s from a verified domain and the overall message alignment is consistent. As long as each signing domain is legitimate and the authentication policies (SPF, DMARC) reflect those relationships, multiple signatures are not only acceptable—they’re a sign of layered trust.

This is the core idea behind industry-standard practices like those outlined in RFC 6376, the technical specification governing DKIM. It allows for multiple signatures as long as they’re cryptographically valid and aligned with the sender’s identity. You may see this in enterprise systems where a marketing platform signs, then a CRM adds its own signature before delivery.

When Multiple Signatures Raise Red Flags

But if you see two DKIM signatures from domains that have no meaningful relationship—say, a customer support address and an unrelated marketing domain—it raises immediate questions. It suggests either poor configuration, a compromised system, or an intentional attempt to obscure the sending source. The lack of alignment between signing domains and the From: or Return-Path: fields breaks DMARC’s alignment requirements, which can trigger filtering or rejection.

DMARC is strict: if a DKIM signature validates but comes from a domain not listed in the Message's From: domain or Return-Path, it fails alignment. In practice, misaligned signatures are a known indicator of spoofing. If you’re doing bulk sending, verify your entire email stack—especially if using third-party tools—to ensure each signature is intentional, legitimate, and properly aligned.

Use tools that validate not just individual addresses, but also the authentication chain. A real-time verification API like the one at MailTester’s API can check if a given email address, when sent via your system, will pass standard checks—including DKIM/SPF/DMARC alignment—before ever hitting an inbox.

The Risk of Multiple DKIM Signatures to Sender Reputation

Multiple DKIM signatures in an email header can raise red flags with inbox providers and spam filters, especially when they’re redundant or not aligned with SPF and DMARC. Receivers may interpret this as misconfigured authentication or an attempt to bypass filtering, which can degrade sender reputation over time. In some cases, spam engines treat overlapping signatures—particularly without proper alignment—as a sign of potential abuse, increasing the risk of inbox placement issues even if the message appears otherwise legitimate.

Why Redundant Signatures Can Backfire

Let’s be clear: having multiple DKIM signatures isn’t inherently wrong—some large senders use them for routing or legacy systems. But when signatures aren’t necessary or are applied inconsistently, they signal poor authentication hygiene. Email receivers, including major providers like Gmail and Outlook, use patterns like this to assess sender trust. If your DKIM records aren’t aligned with SPF or DMARC, or if multiple signatures don’t point to the same domain, it may look like an attempt to obscure sender identity.

According to industry practices documented in RFC 6376, DKIM is designed for single, coherent digital signatures aligned with a sending domain. When multiple signatures exist without clear, valid use cases (like multiple intermediaries forwarding mail with their own signature), the system can be misinterpreted as tampering or misconfiguration. This is especially true when one signature is from a legitimate sender but another comes from a third-party service with no SPF or DMARC alignment. Filtered messages with such anomalies may be held for deeper inspection or marked as suspicious, even without obvious spam content.

How This Impacts Deliverability

Over time, consistently sending emails with multiple DKIM signatures—especially when not aligned or redundant—can lead to subtle reputation penalties. Unlike a hard bounce or DNS block, these signals accumulate without an obvious trigger. An inbox placement test might show low delivery rates even when your list is clean and content is relevant. This happens because spam filtering systems look for behavioral outliers like inconsistent DKIM usage. The absence of clear alignment can make your domain appear less reliable.

Tools like inbox placement testers can help verify how your messages are being received across major providers. If you’re seeing inconsistent results or delivery drops with high-volume sends, review your header structure. Make sure only necessary DKIM signatures are present and confirm they’re properly aligned. For ongoing list maintenance, use a trusted service like bulk email verification to catch malformed or suspicious addresses before they impact your sending reputation.

How MailTester Helps You Detect and Fix Multiple DKIM Signatures

You don’t need to guess when an email has multiple DKIM signatures—MailTester’s real-time verification API scans full headers during delivery testing, flagging inconsistent or redundant signatures. This prevents alignment failures, reduces bounce rates, and strengthens authentication consistency across major inbox providers. With 98.9% accuracy, you get a clear verdict: valid, invalid, risky, or catch-all—based on deep header inspection, not generic filters.

Step-by-Step: How MailTester Detects and Fixes Multiple DKIM Signatures

1. Send the email through MailTester’s inbox-placement test—this mimics real delivery to Gmail, Outlook, Apple Mail, and other major providers. The test examines header structure, including all DKIM-Signature fields, during the validation phase.
2. Inspect header-level anomalies—MailTester parses the full email header to detect multiple DKIM-Signature entries. Multiple or conflicting signatures violate industry-standard authentication practices and can trigger spam filters.
3. Check alignment across SPF, DKIM, and DMARC—each signature must align with the From domain and the sender’s identity. MailTester verifies this in real time, identifying mismatches that often arise when multiple DKIM signatures are present without proper alignment.
4. Review the detailed verdict—results show whether the signature is valid, risky (due to inconsistency), or invalid (due to malformed or conflicting data). The 98.9% accuracy comes from combining header analysis with real-time provider behavior simulation.
5. Take corrective action—use the feedback to audit your email infrastructure. If you're using multiple third-party services (e.g., ESPs, marketing platforms), ensure only one authoritative DKIM signature is present per message, or ensure all are properly aligned and signed.

Why This Matters for Deliverability

Multiple DKIM signatures can confuse email receivers and make it harder to validate trust. While some systems tolerate multiple signatures, many modern filters—especially in Gmail and Outlook—prefer clean, unambiguous authentication. This inconsistency can lower sender reputation, increase bounce rates, and lead to inbox placement drops.

According to the DKIM specification (RFC 6376), a message should have one or more signatures, but they must not conflict in domain or alignment. MailTester checks for this explicitly. If you’re sending bulk mail via platforms like SendGrid or Mailchimp, the presence of multiple DKIM signatures—even if technically allowed—can signal poor configuration.

Use MailTester’s inbox-placement test to simulate delivery with real providers and catch issues like this before your campaign launches. Whether you're validating a single address or auditing a 100,000-list, you get actionable insights, not just a yes/no answer. The tool doesn’t just flag problems—it helps you understand why they happen and how to fix them.

Best Practice: Enforce a Single DKIM Signature Per Email

Only one DKIM signature should be present per email. Multiple signatures confuse email gateways, increase fraud risk, and harm sender reputation. Let’s make sure your sending stack signs only once, and only by trusted systems.

How to enforce a single DKIM signature

• Design your email infrastructure so only your core sending domain (e.g. [email protected]) signs each message. This aligns SPF, DKIM, and DMARC and avoids signature conflicts.
• Do not allow third-party platforms like marketing automation tools or transactional email services to sign unless they’re fully vetted, aligned with your domain policy, and explicitly authorized. Each added signature increases complexity and attack surface.
• Configure your email gateway or MTA to reject or strip DKIM signatures from unapproved sources. Use strict header filtering to reject emails with multiple DKIM signatures unless explicitly allowed.
• Regularly audit outbound email headers using real-time tools. Check for unexpected or unauthorized DKIM signatures during sending. This is especially critical with cloud-based email services or APIs where misconfigurations are easy.
• Use tools like MailTester’s email checker to verify headers before sending. Test deliverability, review DKIM output, and detect anomalies in header structure before blasting.
• Train your team to recognize when a DKIM signature appears from an off-brand domain. A signature from [email protected] on an email from [email protected] is a red flag.

Why multiple DKIM signatures are a problem

Multiple DKIM signatures can appear when third-party systems sign the same message after it’s been sent—the sender, an ESP, or a delivery agent all apply their own signature. This leads to inconsistent validation results, can trip spam filters, and weakens trust signals. Even a single incorrect or malformed signature can cause a legitimate email to be rejected.

According to RFC 6376 (the DKIM specification), a message may carry multiple signatures, but each must be valid and properly aligned. In practice, most receiving systems treat multiple signatures as a sign of poor sender hygiene or potential spoofing. Reputable mail providers like Gmail and Outlook are known to penalize such patterns.

When in doubt about an email’s integrity, check the DKIM record using tools like MxToolbox or Spamhaus—these sites can validate alignment and signature chains.

DKIM, SPF, and DMARC: Their Roles in Email Authentication

You can detect multiple DKIM signatures in an email header when multiple domains or servers sign the same message—common in complex mailing systems with intermediaries like forwarders, ESPs, or resellers. More than one DKIM signature doesn't necessarily mean spoofing; it reflects legitimate multi-hop delivery chains, but it can also indicate attempts to bypass authentication if misconfigured. Use tools that analyze headers to verify each signature’s validity, domain alignment, and chain of trust. To prevent deliverability issues, ensure your setup doesn’t create conflicting or invalid signatures.

The Role of Each Protocol

Let’s break down how SPF, DKIM, and DMARC work together to secure email. Each has a distinct function in email authentication, and they’re often used in combination to verify sender legitimacy.

Each protocol plays a part in verifying email legitimacy. SPF checks sender IPs, DKIM ensures content hasn’t changed, and DMARC enforces policy based on both. Together, they form a reliable triad, though misconfiguration is common—especially when multiple DKIM signatures appear. Multiple signatures may arise when a third party (like a mailing service or forwarder) signs the message. If these signatures don’t align with your domain or aren’t properly validated, your email might be rejected or marked as suspicious.

For example, if a message passes SPF but fails DKIM alignment, DMARC can still fail. According to RFC 7483, proper alignment between From domain and signing domain is essential. Tools like MailTester’s bulk verification can analyze email headers and flag inconsistencies in DKIM signatures, SPF alignment, and DMARC settings, helping you catch issues before sending.

Always validate header chains. Use inbox placement testing to see how your email lands in real inboxes. You don’t need perfect scores—just consistent, clean authentication signals. If you’re seeing multiple DKIM signatures, ensure each one is legitimate and aligned with your sending infrastructure.

How to Test for Multiple DKIM Signatures Using Real Email Data

You can detect multiple DKIM signatures in an email header by sending a real message through MailTester’s inbox-placement test using a live recipient address. The full header output in the test report will show all DKIM-Signature fields. Compare each domain to your authorized sending domains—any mismatched or unnecessary signatures should be removed to avoid alignment issues that trigger spam filters.

Step-by-Step Header Inspection Process

1. Send your email through MailTester’s inbox placement tester using a verified, active email address. This ensures you’re testing against real-world delivery conditions, including how receiving servers parse and validate headers.
2. Once the test completes, open the full header report. Look for multiple DKIM-Signature fields. Each one appears as a separate header line, typically beginning with Dkim-Signature: followed by parameters such as d= (domain) and s= (selector).
3. Check the d= value of each signature. If it points to a domain not part of your trusted sending ecosystem—like a third-party tool, outdated partner, or a domain no longer in use—this is a red flag. Multiple signatures from unverified sources can break DKIM alignment and reduce deliverability.
4. Validate that each domain in the DKIM-Signature header matches a domain currently publishing a valid DNS record with a public key. Use tools like MXToolbox or RFC 6376 to confirm DKIM record integrity and policy compliance.
5. If you find non-compliant or outdated signatures, either remove them from your sending stack or reconfigure your email system to only sign with domains you control. You can use MailTester’s bulk verification tool to clean up your sender list before deploying such changes.

Why This Matters

Multiple DKIM signatures are not inherently invalid—but when they come from untrusted or mismatched domains, they cause alignment failures. Receiving servers apply DKIM alignment checks (as defined in RFC 6376) to determine if the signing domain matches the From domain. If not, spam filters may treat the message as suspicious.

Even a single misaligned signature can hurt inbox placement. Let’s say your marketing platform signs with d=marketing.sendgrid.net, but you didn’t include that domain in your SPF or DMARC policy. The email may be rejected or marked as spam, even if the content is clean.

Aligning your DKIM signatures with your sending infrastructure is an industry-standard practice for maintaining sender reputation.

Final Thoughts: Preventing Header Confusion Before It Affects Your Inbox Placement

Multiple DKIM signatures in an email header are rarely the direct cause of a block. They are, however, a strong signal that your email infrastructure has misconfiguration or overlapping signing policies.

These anomalies can confuse email receivers, complicate authentication checks, and increase the risk of deliverability issues over time—especially when combined with other signals like inconsistent SPF or mismatched headers.

Using MailTester’s real-time verification and bulk list checks helps you identify and fix such issues before they impact sender reputation or inbox placement.

Keep your email setup simple: one authenticated sender, one signing domain, and one clear policy. Clean headers reduce ambiguity and increase trust.

Sources

• DMARC adoption among top domains surged 75% between 2023 and 2025 — from 27.2% to 47.7% — in the wake of Google and Yahoo's bulk-sender authentication requirements. — EasyDMARC 2025 DMARC Adoption Report (2025)
• Since May 5, 2025, Microsoft Outlook requires SPF, DKIM, and DMARC from domains sending 5,000+ emails per day, rejecting non-compliant mail outright at the SMTP level with error 550 5.7.515. — Microsoft Outlook requirements (via MailOver bulk-sender requirements guide) (2025)

Keep reading

• Email authentication: SPF, DKIM, DMARC, BIMI and MTA-STS (complete guide)
• Monitoring Email Deliverability Performance After DMARC Enforcement Shifts
• Troubleshooting SPF Records with DNSSEC Enabled in 2026
• How to Correlate Bounce Codes with DMARC Failure Reports in 2026
• DMARC Report Delivery Blocked by DNSSEC Configuration Flaws in 2026

Ready to put this into practice? MailTester verifies emails with 98.9% accuracy — start with 100 free verifications.

Frequently asked questions

Can multiple DKIM signatures cause an email to be rejected?

Not usually outright, but multiple signatures can reduce trust with some filters, especially if alignment with SPF or DMARC fails. They may lead to a higher spam score.

Is it normal to see multiple DKIM signatures from a single sender?

No, it’s not normal. A single authorized sender should sign once. Multiple signatures suggest misconfiguration or unapproved intermediaries.

How does MailTester detect multiple DKIM signatures?

Through full header analysis during inbox-placement and real-time verification tests, identifying and reporting multiple DKIM-Signature fields.

Does having multiple DKIM signatures break DMARC alignment?

It doesn’t break DMARC outright, but it makes alignment difficult to validate. Only one DKIM domain can be aligned with the From: domain at a time.

Can content delivery networks add DKIM signatures?

Yes, some CDNs or proxy systems add DKIM to secure content delivery. If not properly coordinated, this can add extra signatures without alignment.

Do all email providers check for multiple DKIM signatures?

Most do not explicitly reject emails for multiple signatures, but they may use them as part of spam scoring and reputation analysis.

How often should I audit my email headers for multiple DKIM signatures?

Audit before large campaigns, after system changes, or if you notice sudden delivery drops. Use MailTester’s inbox-placement test for ongoing checks.

What’s the impact of multiple DKIM signatures on deliverability?

It can signal poor infrastructure hygiene, leading to increased scrutiny, higher spam scores, and lower inbox placement—especially if not aligned.

Can I keep multiple DKIM signatures if they’re all valid?

Technically yes, but only if every signature is from a trusted, aligned sender domain. Otherwise, it increases risk without benefit.

Does MailTester flag every DKIM signature in a header?

Yes. Our verification includes full header parsing and reports the count, domains, and alignment of every DKIM-Signature field found.

Can a catch-all email address cause multiple DKIM signatures?

No. Catch-all addresses don’t cause additional signatures. They may hide invalid addresses but don’t affect email header structure.

What should I do if I find multiple signatures in a test?

Identify the sender origins of each signature. Retain only one from a valid, aligned domain. Reconfigure systems to prevent redundant signing.