Relaxed Body Canonicalization Setup

Why does body canonicalization impact email deliverability?

You send an email, it passes SPF and DKIM just fine — but then it lands in spam, or vanishes entirely. You check the headers. Everything looks correct. What’s really going on?

One subtle but critical factor is body canonicalization. It dictates how an email’s content is treated during DKIM signing and verification. If it’s too strict, even a single space or line break added during transit can break the signature — and your email gets rejected.

Relaxed body canonicalization allows minor, harmless formatting changes without invalidating the DKIM signature. That means better delivery reliability, especially when your email passes through multiple servers or filtering systems. This is how to configure relaxed body canonicalization for better email deliverability.

Key takeaways

DKIM signatures can fail if body canonicalization is too strict, even for minor content changes during transit.
Relaxed body canonicalization preserves DKIM validity by tolerating insignificant formatting differences.
Configuring relaxed body canonicalization improves inbox placement for emails that undergo routine routing or processing.

What is relaxed body canonicalization in DKIM?

Relaxed body canonicalization is a DKIM standard that ignores minor changes to whitespace and line breaks in an email’s body when validating the digital signature. This means a signed email can still pass verification even if the server or client re-formats the content—like adding line breaks or trimming spaces—without breaking the signature. It’s especially important for HTML emails, dynamic templates, or any message generated programmatically where formatting changes are common.

Why it matters for email deliverability

When email servers process messages, they often tweak formatting—inserting line breaks, adjusting indentation, or compressing whitespace—especially when handling HTML content. Without relaxed body canonicalization, these small changes would invalidate the DKIM signature, leading to fails and lower deliverability. This is why the DKIM specification allows for relaxed canonicalization: to ensure signatures remain valid across real-world delivery scenarios.

For example, imagine your marketing email uses a template with embedded CSS and dynamic text. A mail server might reflow the HTML or normalize whitespace during processing. If you’re using strict body canonicalization, that slight change could break the signature, triggering a fail and possibly marking your email as suspicious. With relaxed body canonicalization, the signature still checks out—your email stays trusted.

This behavior is defined in RFC 6376, the standard for DKIM, which explicitly supports relaxed header and body canonicalization. The relaxed approach is the default in practice because most mail systems modify message content during delivery. You can check a message’s DKIM status using tools like MxToolbox or Spamhaus, which analyze headers and signatures to detect alignment and validity.

Let’s say you’re debugging a sudden spike in bounces or rejections. One cause might be a missing or misconfigured DKIM record. Tools like MailTester’s verification API or bulk verification can help spot invalid or misaligned emails before they go out—catching delivery issues earlier, including problems related to DKIM configuration.

How does relaxed body canonicalization improve email deliverability?

Relaxed body canonicalization reduces DKIM failures caused by harmless changes in email formatting during transit—like whitespace adjustments or line breaks—keeping your authentication intact. This means fewer rejected messages, better sender reputation, and higher chances your emails land in the inbox, not the spam folder. It’s a small but meaningful fix for consistent deliverability across platforms.

Why formatting changes break DKIM (and how relaxed body helps)

When emails move through servers, tiny changes—like adding a line break or reordering attributes—can appear as a signature mismatch to strict DKIM checks. This triggers a failure, even if the message content is unchanged and legitimate. Relaxed body canonicalization ignores these minor, non-malicious differences during the signature validation process, so your email still passes.

For example, an email sent from a marketing platform might undergo reformatting before hitting an inbox. Strict canonicalization would flag this as tampering. But relaxed body lets those innocuous shifts pass, preserving the DKIM signature without false positives.

Impact on sender reputation and inbox placement

Every DKIM failure, even a temporary one, adds friction to your sender reputation. ISPs like Gmail and Outlook track these errors across large volumes of emails. High failure rates signal instability or poor sending practices, which can lead to throttling or filtering.

By reducing unnecessary DKIM failures, relaxed body canonicalization supports a cleaner, more consistent authentication record. This improves your email’s long-term reputation, which directly influences inbox placement. You’re not just avoiding delivery errors—you’re building trust with receiving servers.

It also ensures consistency across email clients and platforms. Whether your email is opened in Gmail, Outlook, or a mobile client, the same authentication logic applies. This uniformity helps avoid edge cases where one client accepts a message but another rejects it due to a misaligned signature.

Relaxed body canonicalization is not a substitute for proper email setup, but it’s a crucial layer when working with dynamic or third-party tools that modify content during transit. It’s an industry-standard practice documented in RFC 6376, the base spec for DKIM.

If you're sending at scale or using multiple platforms, verify your list hygiene and authentication setup regularly. MailTester’s bulk verification checks for invalid or high-risk addresses, while its inbox placement test helps validate how well your emails perform across real inboxes. A clean list and reliable authentication go hand in hand.

How to set relaxed body canonicalization in your email system

You can improve email deliverability by setting body canonicalization to 'relaxed' in your DKIM configuration. This ensures consistent signing despite minor formatting changes in email clients or forwarders, reducing the risk of signature failures. Most major platforms support this setting, and it’s a widely recommended practice for maintaining reliable authentication.

Step-by-step: configure relaxed body canonicalization

Access your email service’s DKIM settings—whether you're using SendGrid, Mailgun, AWS SES, or a self-hosted MTA like Postfix or Exim. This is typically found under security, domain authentication, or mailing settings.
Navigate to the DKIM policy or signing configuration section. Look for options labeled "body canonicalization" or "canonicalization method." Not all platforms expose this directly, but it’s usually configurable in advanced settings.
Set the body canonicalization method to relaxed. This allows the signature to remain valid even when line breaks, whitespace, or ordering changes occur during transit or client rendering.
If using a custom MTA such as Postfix or Exim, edit your DKIM signing configuration file (e.g., /etc/opendkim/KeyTable or signing.conf). Ensure the BodyCanonicalization directive is set to relaxed in your policy definition.

Why relaxed body canonicalization matters

Many email systems normalize content during transit—especially in forwarding chains or when rendered in clients like Gmail or Outlook. If your DKIM signature uses strict body canonicalization, even small changes can break the signature, triggering rejection by receiving servers.

The 4 steps described in “Step-by-step: configure relaxed body canonicalization”, in order.

The RFC 6376 defines relaxed body canonicalization as a standard method to handle these variations. Most reputable systems support it, and it's considered an industry best practice for maintaining deliverability under real-world conditions.

Testing your configuration with tools like inbox placement tests can confirm whether your DKIM signatures remain valid across multiple inboxes and filtering systems. Consistent signature validation reduces the risk of messages being marked as spam or blocked entirely.

Remember: even small misconfigurations in signing policies can cause widespread delivery issues. Use bulk email list verification to audit your sender infrastructure and catch deliverability risks early—before they impact your campaigns.

What happens if you use strict body canonicalization?

Using strict body canonicalization can break your DKIM signature with even minor changes—like a single space or line break—leading to signature failures. When this happens, receiving servers may reject your email outright or mark it as suspicious, especially if the failure occurs during multiple message hops. Over time, repeated failures hurt deliverability and erode sender reputation.

How strict canonicalization harms DKIM integrity

DKIM relies on the consistency of the message body during signing and verification. Under strict body canonicalization, even a single character change—like a line break inserted by a mailing list or a content filter—breaks the hash alignment. Since the receiver recalculates the hash and compares it to the signed value, any mismatch invalidates the signature.

For example, a plain-text email with a newline added during transit fails DKIM validation. This isn’t rare: in practice, many email systems modify message content during relays—adding tracking headers, rewriting links, or injecting footers. In these cases, strict canonicalization makes compliance nearly impossible.

If the DKIM signature fails, receiving servers often treat the message as untrustworthy. Depending on the policy, the email may be rejected, sent to spam, or throttled. The more times this occurs, the stronger the signal to ISPs that your sending behavior is inconsistent or potentially manipulative.

Long-term impact on sender reputation

Sender reputation isn’t just about bounce rates or spam complaints—it’s also built on technical reliability. Consistent DKIM failures, even if caused by configuration, are a red flag to modern filtering systems. Services like Spamhaus and MXToolbox track such patterns and use them to assess trustworthiness.

Even a single failure in a high-volume send can trigger suspicion. Over time, repeated failures—especially across multiple hops—can reduce inbox placement rates, even for legitimate mail. This isn’t just theoretical; RFC 6376 (the DKIM standard) explicitly warns that improper canonicalization undermines the entire mechanism.

Let’s be clear: strict body canonicalization is not practical for most senders. It works only in perfectly controlled environments where no processing occurs between signing and verification. For real-world email delivery, relaxed body canonicalization is the industry-standard choice.

Check your setup before sending. Use a tool like MailTester’s email checker to validate addresses and test deliverability before you send to large lists.

DKIM body canonicalization standards and compliance

DKIM uses body canonicalization to standardize message content so signatures remain valid across transit. RFC 6376 defines two methods: 'simple' (preserves all whitespace) and 'relaxed' (normalizes line breaks and trims extra spaces). You should use 'relaxed' body canonicalization to improve compatibility with email services and reduce signature mismatches during delivery.

The role of relaxed body canonicalization

Relaxed canonicalization strips leading and trailing whitespace, converts line endings to CRLF, and treats sequences of spaces as single spaces. This ensures DKIM signatures stay intact even when email clients or intermediaries modify formatting slightly. Without it, minor changes—like adding a newline or reflowing text—can break the signature and cause delivery rejection.

Most modern email providers, including Gmail, Outlook, and Apple Mail, expect relaxed body canonicalization. If you use 'simple' or don't apply canonicalization consistently, your messages may fail signature validation even if they're otherwise legitimate. This leads to misclassified emails and degraded sender reputation over time.

Standards compliance and real-world impact

According to RFC 6376, the relaxed method was designed to accommodate common email transformations without invalidating signatures. It’s the established norm across the industry, and failing to follow it reduces the chances that your email will pass authentication checks at scale.

Many bulk senders overlook canonicalization settings when configuring DKIM, treating them as implementation details. But even small misconfigurations—like choosing 'simple' when 'relaxed' is expected—can result in high bounce rates or spam placement. Regular verification tools can catch these issues early.

Let’s test your DKIM setup with a real inbox placement test. A single address test can verify whether your domain’s headers and body are being processed correctly during delivery. If you're managing a large mailing list, bulk verification ensures all addresses maintain a clean, compliant format across your entire list.

For accurate, real-time checking, use MailTester’s email checker to validate individual addresses before sending. To review full list health and catch issues like broken DKIM setup, explore the bulk verification tool. Both integrate directly with major platforms like Mailchimp, HubSpot, and Klaviyo through our integrations.

While relaxed canonicalization is technically optional, it’s functionally required for consistent delivery. Deviating from the norm invites authentication failure. Stick with relaxed body canonicalization—it’s the standard for a reason.

How to test if your DKIM signature is configured correctly

You can verify your DKIM configuration by sending a test email through a real-time verification service, checking the email headers for a DKIM-Signature field with c=relaxed and a b= value, then validating the signature using open-source tools like DKIM Validator or DNSDKIM.com. This ensures your messages are trusted by receiving servers and not silently dropped.

Send a test email via a real-time verification service — Use a tool like MailTester’s email checker to send a test message from your domain. This simulates real-world delivery and captures full headers for analysis. You're not testing deliverability yet — you're testing whether the signature exists and is properly structured.
Inspect the email headers for the DKIM-Signature field — Open the full headers of the received email (usually via your mail client’s “Show Original” or “View Source” option). Look for a line starting with DKIM-Signature:. It must contain c=relaxed — this is essential for proper canonicalization and compatibility with modern mail servers.
Confirm the presence of the b= parameter — The signature should include a b= field containing the actual cryptographic signature. If this is missing or empty, the signature is invalid. A valid DKIM-Signature line typically looks like: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed; d=yourdomain.com; s=selector; bh=...; b=....
Validate the signature with open-source tools — Tools like DKIM Validator or DNSDKIM.com accept raw headers or a public key and verify if the signature matches the expected hash. They’ll tell you if the domain, selector, or hash values are incorrect.
Check the DNS TXT record for the selector — Use DNSDKIM.com or a command-line tool like dig to confirm that your DKIM public key is published in your domain’s DNS under the correct selector (like selector1._domainkey.yourdomain.com). Missing or malformed records are a common cause of failure.

Why relaxed canonicalization matters

Without c=relaxed, even small formatting differences in the email’s body (like whitespace or line breaks) can invalidate the signature. This is why relaxed body canonicalization is required for compatibility with most modern email providers. As defined in RFC 6376, this allows some flexibility while still preserving security. Ignoring it leads to inconsistent results across receivers.

When to retest

Always retest after changing your DKIM keys, updating DNS records, or switching email infrastructure. Even minor delays in DNS propagation can cause failures in signature validation. Use MailTester’s inbox placement test to verify both signature integrity and final inbox delivery.

Why MailTester helps prevent deliverability issues from DKIM misconfigurations

You don’t need to guess if your DKIM setup is working. MailTester’s inbox-placement tests validate your full email authentication stack—SYNTAX, alignment, and real-world deliverability—before you send. This reduces the risk of bounced or marked-as-spam messages caused by overlooked DKIM errors.

Testing what matters: real delivery behavior, not just syntax

Many tools only check if your DKIM signature exists or if it passes basic syntax. That’s not enough. Malformed or misaligned DKIM headers can still break deliverability—even if they validate in a lab. MailTester goes further: it simulates delivery to Gmail, Outlook, and Yahoo using actual infrastructure, testing how recipients’ servers respond in real time.

With inbox-placement testing at your fingertips, you learn whether your emails land in the inbox—or end up in the spam folder—based on your DKIM alignment, SPF, and DMARC policies. This is the most realistic form of pre-send validation available.

Proactive verification ensures consistent authentication

Let’s say you’re using a third-party sender like SendGrid or Mailchimp. Their DKIM signing might fail silently, especially across different domains or mailing lists. A misconfigured selector, expired key, or incorrect TXT record can all cause delivery failures—but you won’t know unless you test properly.

MailTester’s real-time API checks more than just address format. It validates whether your DKIM, SPF, and DMARC settings are aligned and consistently applied. You can integrate this directly into your sending workflow using the verification API to catch issues before they hit the inbox.

For bulk lists, run a bulk verification to identify all addresses with authentication gaps. This helps you clean lists proactively and avoid sending to domains where your DKIM fails to align—especially critical when you’re managing high-volume campaigns.

Even minor issues, like a missing or malformed DKIM signature, can lead to rejection by providers like Microsoft or Google. By aligning your configuration with standards—like those in RFC 6376—you ensure your emails are trusted, not blocked.

DKIM isn’t a checklist item. It’s a gatekeeper. MailTester treats it as one.

Common pitfalls when setting up DKIM with relaxed body canonicalization

Switching to relaxed body canonicalization can improve deliverability by allowing minor content changes during routing, but it’s easy to break things. Forgetting to update your DKIM records, relying on outdated docs that default to strict mode, or assuming all platforms support relaxed mode can invalidate your signature or trigger bounces. Always verify your setup matches the actual email flow.

Don’t assume your DKIM setup is consistent across platforms

Update your DKIM record whenever you change the canonicalization method—using strict on one platform and relaxed on another will cause signature mismatches, even if the domain and selector are the same.
Check if your email service provider (ESP) or mail transfer agent (MTA) supports relaxed body canonicalization. Some older systems still require strict mode; if you’re using a legacy platform, verify its behavior in the DKIM RFC.
Don’t rely on documentation that defaults to strict mode—many guides still reference older implementations. Look for explicit mention of "relaxed body canonicalization" in your ESP’s mail flow or DKIM configuration section.

Verify your configuration in real-world conditions

After updating your DNS record, test the DKIM signature with actual email delivery. Use tools to simulate inbound delivery and validate the signature against the full message path.
Some ESPs may adjust headers during transit (e.g., adding tracking tags or rewriting URLs), which relaxed body canonicalization is designed to handle. But if your signature was generated with strict mode, these small changes will cause failure.
Use inbox placement testing to see how your DKIM signing holds up across major providers. An email that passes technical validation may still land in spam if the canonicalization isn’t aligned with the receiving server’s expectations.

Relaxed body canonicalization isn’t a fix-all—it’s a precision tool. The real win comes when it’s correctly applied across your entire email infrastructure. Use inbox-placement testing to validate how your DKIM setup performs in practice, before sending to live recipients.

Best practices for maintaining consistent DKIM and deliverability

You achieve reliable email deliverability by consistently applying relaxed body canonicalization in DKIM signatures, ensuring your messages aren't broken by minor formatting changes during transit. Monitor DKIM verification status through inbox testing and header analysis, and use email verification tools like MailTester to catch invalid or misconfigured addresses and sending setups before they hurt sender reputation.

Apply relaxed body canonicalization by default

Always use relaxed body canonicalization unless your email infrastructure requires strict mode—most modern mail systems expect relaxed.
Relaxed canonicalization tolerates minor whitespace or line break differences in the message body, preserving DKIM validity across routing hops.
Using strict canonicalization increases the chance of signature failure when headers or body formatting shift during transit, even subtly.
Check your existing DKIM configuration against RFC 6376, Section 3.7 to confirm you're applying the correct method.

Verify and audit sender setup health

Test deliverability by sending real messages to known inboxes and checking DKIM and SPF results in the full message headers.
Use inbox placement testing tools to simulate real-world delivery paths and check if DKIM signatures remain valid through multiple forwarding or filtering layers.
Run regular bulk email list verification using MailTester’s bulk verification to identify invalid, role-based, or disposable addresses before sending.
Integrate the MailTester API into your onboarding or campaign processes to catch poor-quality addresses in real time.
Monitor sender reputation through third-party feedback loops and blocklist monitoring services like Spamhaus to detect patterns of failure.

DKIM is only as strong as its consistent application. A single misconfigured signature can damage trust with receivers that enforce strict authentication checks.

Final step: validate your email flow end-to-end

Authentication checks alone don’t guarantee inbox placement. Real-world testing confirms whether relaxed body canonicalization and other configurations work in practice.

Test your delivery path with inbox-placement analysis

Use MailTester’s inbox-placement testing to send real emails from your production system to live inboxes. This reveals how your messages appear to recipients and intermediaries like ISPs and spam filters.

Inspect alignment and signature status

Review the test results for DKIM, SPF, and DMARC alignment. Look for authentication failures—especially DKIM signature validation errors under relaxed body canonicalization rules. These often indicate signature mismatch due to header or body normalization differences.

Adjust configuration based on findings

Fix misalignments by refining your DKIM signing setup. Ensure your email client or service applies relaxed body canonicalization correctly. Test again until both alignment and signature validation succeed.

Keep reading

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

Frequently asked questions

What is body canonicalization in DKIM?

It’s the method used to normalize the email body during DKIM signature creation and verification. 'Relaxed' ignores minor formatting differences like whitespace and line breaks.

Should I use relaxed body canonicalization?

Yes—unless you're working with a system that mandates strict mode. Relaxed is the standard for modern email delivery and reduces signature breakage.

Can relaxed body canonicalization cause security risks?

No. It only affects whitespace and line endings—common in email rendering. It does not impact message content integrity.

Does SendGrid support relaxed body canonicalization?

Yes. SendGrid uses relaxed body canonicalization by default in its DKIM implementation for better deliverability.

How do I check if my DKIM signature uses relaxed body canonization?

Inspect the DKIM-Signature header in the email. If the value includes `c=relaxed`, then relaxed body canonicalization is in use.

What happens if DKIM fails due to strict body canonization?

The email may be rejected or flagged as suspicious, reducing inbox placement and harming sender reputation over time.

Can MailTester detect incorrect body canonicalization settings?

Yes. MailTester’s inbox-placement tests check DKIM alignment and signature integrity, flagging issues arising from misconfigured canonicalization.

Do all email providers support relaxed body canonicalization?

Most major providers accept relaxed body canonization. Some legacy systems may enforce strict mode, but this is rare in modern infrastructure.

Should I reconfigure DKIM after switching to relaxed body canonization?

Yes. After updating the canonicalization method, you must regenerate and publish the DKIM DNS record to ensure validation works.

Is relaxed body canonicalization mandatory for SPF and DMARC?

No. SPF and DMARC don’t use body canonicalization. Relaxation only applies to DKIM signatures.

How often should I test my DKIM setup?

Test at least monthly or after any change to your email infrastructure. Use MailTester’s real-time API for ongoing validation.

Can I use MailTester to test DKIM with relaxed body canons?

Yes. MailTester supports full inbox-placement testing, including DKIM validation with relaxed body canonicalization.