Why rsa-sha1 in DKIM is no longer safe for email deliverability

You’re sending transactional emails, and your inbox placement is dropping. You’ve checked SPF, verified your domain, even warmed up your IP. But the deliverability team still sees red. Could your DKIM signature be the silent culprit?

Behind the scenes, your email signature uses a cryptographic algorithm called rsa-sha1—one once widely accepted, now flagged as insecure. Major mailbox providers like Google, Yahoo, and Microsoft have stopped accepting messages signed with it. If your email still uses rsa-sha1, you’re risking rejection, poor sender reputation, and higher chances of ending up in spam or junk folders.

Knowing how to update DNS DKIM record to remove rsa-sha1 dependency isn’t just a technical task—it’s a deliverability necessity. Ignoring it means you’re exposing your sender reputation to avoidable risk. This guide walks through why the change is essential and exactly what you need to do to modernize your DKIM setup safely and correctly.

Key takeaways

  • rsa-sha1 is deprecated due to known cryptographic vulnerabilities and is no longer supported by major inbox providers.
  • Messages signed with rsa-sha1 are increasingly rejected by Gmail, Yahoo, and Outlook, directly harming inbox placement.
  • Updating your DKIM DNS record to use rsa-sha256 or rsa-sha512 is required to maintain sender reputation and ensure long-term deliverability.

How to update your DNS DKIM record to remove rsa-sha1 dependency

You can remove the rsa-sha1 dependency from your DKIM record by generating a new key pair using rsa-sha256 or another SHA-2 algorithm, updating your DNS TXT record with the new public key while keeping the selector and domain unchanged, and then enabling the new key in your email service provider. This ensures ongoing compliance with modern email security standards and avoids delivery issues caused by deprecated algorithms.

Step-by-step: Update your DKIM record

  1. Log in to your DNS provider’s dashboard — Access your account at Cloudflare, AWS Route 53, GoDaddy, or your provider of choice. This is where you’ll manage your domain’s DNS records.
  2. Locate your existing DKIM TXT record — Look for a record named _domainkey or selector1._domainkey (name varies). These are common for DKIM setup across email platforms.
  3. Check if the record uses rsa-sha1 — Open the record’s value and look for rsa-sha1 in the signature part. If present, it’s outdated. The IETF has deprecated this algorithm due to cryptographic weaknesses (see RFC 8301).
  4. Generate a new DKIM key pair with rsa-sha256 — Use your ESP’s key generation tool (like SendGrid or Mailgun’s admin interface) or a secure tool like OpenSSL to create a key pair. Prioritize rsa-sha256 or any SHA-2 variant. This ensures long-term compatibility and stronger authentication.
  5. Update the TXT record with the new public key — Replace only the key string in the record’s value. Keep the selector (e.g., selector1) and domain unchanged. The selector must match what your email platform expects.
  6. Enable the new key in your ESP — Go to your email service provider (SendGrid, Mailgun, etc.) and switch to the new DKIM key. Do not disable the old one immediately — keep it active until you've verified delivery.
  7. Verify the new record works — Use a DNS lookup tool like MxToolbox or the command line (dig TXT _domainkey.yourdomain.com) to confirm the record returns the updated key and algorithm.
  8. Monitor deliverability for 2–3 days — Watch inbox placement, open rates, and bounce reports. Some ISPs may take time to update their evaluation of your domain’s authentication. Use tools like inbox placement tests to validate delivery results across providers.

Why this matters for email reliability

Algorithms like rsa-sha1 are no longer considered secure by industry standards. As more providers phase them out, messages with outdated keys may be rejected or marked as suspicious. Updating to rsa-sha256 ensures your messages continue to pass authentication checks and maintain sender reputation. It also future-proofs your email infrastructure.

What a modern DKIM record looks like in DNS

You can update your DKIM record to remove rsa-sha1 dependency by ensuring it uses a modern key format: v=DKIM1; k=rsa; p=... with a hash algorithm like sha256 or higher. The record must include the k=rsa tag and use a public key that supports strong hashing. Validating the DNS record with tools like MxToolbox or verifying it via email authentication checkers ensures it’s recognized by modern mail systems.

Breaking down the components

Modern DKIM records require the v=DKIM1 tag to identify the version, and k=rsa to specify the key type. The p= value contains the public key in PEM format, and it must be generated using a secure, non-deprecated hash function. Most email providers now reject or flag messages signed with weaker algorithms.

Don’t rely on outdated methods. A record like h=sha1 or o=rsa-sha1 is no longer acceptable. Instead, use h=sha256 or h=sha256+sha256 as the hash specification. For context, the IETF’s RFC 8301 explicitly deprecates weak algorithms like SHA-1 in email authentication, emphasizing the shift toward stronger cryptographic standards [RFC 8301].

Validation is non-negotiable

Even if your record looks correct, it won’t work if it doesn’t pass DNS-level checks. Use a real-time DNS lookup tool to confirm the record is published and resolvable. Then, test the full end-to-end flow with an inbox placement tool to ensure the signature is recognized by major mail providers.

For teams verifying bulk lists or testing sender health, MailTester’s inbox placement testing can validate how well your DKIM signature performs in real-world inboxes. It checks not only the record syntax but also how receivers interpret it—something generic DNS tools miss.

Always verify the record’s structure and test it across multiple mail platforms. Your mail server won’t know if the record is broken, but a real-world test will. That’s how you know it’s ready.

How to verify that your DKIM record no longer uses rsa-sha1

You can confirm your DKIM record no longer uses rsa-sha1 by checking the DNS TXT record for your domain’s DKIM selector, looking specifically for the p= value and scanning for rsa-sha1 or sha1. If you find them, the record is still vulnerable. A valid record should use rsa-sha256 or another SHA-2 algorithm. Always double-check with a public tool or monitor delivery logs for failures.

Check your DKIM TXT record directly

  • Use a DNS lookup tool like MxToolbox’s DKIM Checker or run dig +short TXT _domainkey.yourdomain.com in your terminal.
  • In the output, locate the public key field marked with p= — it begins with v=DKIM1; k=rsa; p=.
  • Scan the full value after p= for rsa-sha1 or sha1. If either appears, the record is outdated and insecure.
  • Confirm the record uses rsa-sha256 or rsa-sha512. These are the current industry-standard algorithms as defined in RFC 8301.

Validate beyond DNS — test delivery and logs

  • After updating, send a test email from your domain and use tools like MailTester’s Inbox Placement Checker to see if it clears authentication.
  • Inspect your email service provider’s logs for errors related to DKIM signature verification or algorithm mismatches.
  • Check if receivers are rejecting messages due to deprecated signature methods — this is common with older mail servers or strict filtering setups.
  • If you’re unsure whether a domain is correctly configured, run a full bulk list verification on your sending list to catch any malformed or incorrectly configured emails.
The shift from SHA-1 to SHA-2 is not optional. Major providers like Google and Microsoft now actively reject messages using weak cryptographic algorithms.

You’re not just updating a record — you’re ensuring your messages survive the modern inbox war. Legacy algorithms are no longer tolerated by the largest email platforms. If your DKIM record still relies on rsa-sha1, your delivery rate could be silently suffering. Use the tools above to verify every change before going live.

Common mistakes when updating DKIM records

Updating your DKIM record isn't just about pasting a new key—it’s a precise process. The most common failures include overwriting the full TXT record, using a selector mismatch, forgetting to re-enable DKIM in your mail system, or deploying a malformed key. These errors break authentication and can spike bounces or trigger spam filters. Even small mistakes can degrade sender reputation and hurt deliverability.

Overwriting the entire TXT record

  • Don’t replace the whole record—only update the public key part. The selector and DNS syntax must stay intact. Overwriting the complete TXT record risks removing critical metadata like the selector or version.
  • Using RFC 6376 as a reference helps ensure you keep the correct record structure—especially the selector._domainkey.example.com format and proper field separation.

Selector and key misalignment

  • If you change the selector (e.g., from default to newkey), you must regenerate the private key and update it in your sending platform. A mismatch between the selector in DNS and the one configured on your mail service breaks DKIM validation.
  • Mail servers check both the selector and the key. A mismatch—even in a single character—results in a failed signature and likely rejection.

Missing re-enablement in the sending system

  • Some platforms require you to manually turn DKIM back on after adding a new key. If you generate a new key but leave DKIM disabled in your ESP (like SendGrid, Amazon SES, or Mailchimp), no signing occurs, despite a working DNS record.
  • Check your provider’s dashboard or API settings to ensure the new key is actively used. If you’re using an automation tool, make sure the key isn’t overwritten during sync.

Malformed or invalid key format

  • Digital signatures depend on correct Base64 padding and formatting. Missing padding or improper newlines in the public key can cause signature failures even if the key looks correct.
  • Use a tool like RFC 6376 or a DKIM validator to spot errors in field parsing. Tools like MailTester’s email checker can test a single address’s authentication readiness before scaling.

Skipping pre-deployment testing

  • Don’t retire the old key before verifying the new one works. Send a test email from a known-good address and check its headers for a valid DKIM-Signature. Use MXToolbox’s DKIM checker to validate the DNS record independently.
  • Delay the key rollover until you confirm the new key passes both DNS lookup and email header validation. A rushed switch breaks signing and can take hours to reverse.

How to avoid breaking email delivery during DKIM updates

You reduce disruption risk by keeping your old DKIM key active for at least 72 hours after deploying a new one, using a secondary selector like selector2 during the transition, testing sample mail with the new key before full rollout, monitoring bounce rates and spam complaints afterward, and validating sender reputation with real-time verification tools. This prevents broken authentication chains and ensures deliverability stays stable.

Step-by-step transition protocol

  • Deploy your new DKIM key with a distinct selector (e.g., selector2) to avoid conflicts with the current key.
  • Keep the old key active for at least 72 hours after the new one is live to allow mail transit time across different mail servers.
  • Use a tool like MailTester’s inbox placement tester to send a sample message using the new key and confirm it arrives in the inbox, not spam, without authentication errors.
  • Monitor bounce logs and spam complaint reports for the first 72 hours post-update—sudden spikes signal misconfiguration.
  • Ensure your email provider or ESP supports the new signature algorithm; RFC 8446 outlines modern cryptographic standards, including deprecation of rsa-sha1.

Proactive verification and monitoring

DKIM failures can go unnoticed if you’re not testing with real-world conditions. Relying solely on DNS propagation checks isn’t enough—mail servers evaluate signature validation in context.

  • Before launching with full volume, run a bulk verification of your recipient list using the MailTester bulk verification tool to catch invalid or non-deliverable addresses.
  • Use a real-time email verification API to test if your sender reputation remains healthy after the update.
  • Check for issues like mismatched SPF records or DMARC policies that could compound delivery problems.
  • Monitor your IP and domain reputation with tools that track blocklist entries and sender score trends.
Deliverability doesn’t break overnight—it degrades silently. Monitoring for subtle shifts in bounce rate, spam complaints, and inbox placement is more effective than just fixing the DKIM key.

Why email verification helps after DKIM changes

After updating your DKIM record to drop rsa-sha1, you need to verify your email list to catch any addresses that failed revalidation or were incorrectly flagged during the transition. Old, role-based, or disposable emails often slip through automated systems, and without verification, they can hurt deliverability even after stronger authentication is in place. Use bulk list verification to clean your list before sending, ensuring only valid addresses remain.

Check for lost or misclassified addresses

DKIM changes can trigger revalidation across email providers, and some legitimate addresses might temporarily fail due to caching or slow propagation. Let’s be clear: no system is perfect. If your email list wasn’t verified after the update, you may have sent to outdated or inactive addresses—ones that now bounce or get marked as spam.

Using tools like MailTester’s bulk email verification helps catch these cases before they damage your sender reputation. It checks for roles (like admin@, sales@), disposable domains, and invalid syntax, which often get overlooked during technical setup.

Ensure deliverability gains aren’t offset by bad data

Improving your DKIM signature is a step forward—but if your list contains high-risk or invalid addresses, your inbox placement may still suffer. Spam filters don’t just look at technical headers; they also analyze sender behavior and list hygiene.

According to data from Return Path’s [2022 Email Sender and Provider Performance Report](https://www.returnpath.com/resources/2022-email-sender-and-provider-performance-report/), senders with clean lists see higher inbox placement, even when using modern authentication. But the benefit is lost if you’re still hitting non-existent or role accounts.

MailTester’s 98.9% accuracy rate means you’re less likely to discard valid addresses while removing risky ones. The system distinguishes between temporary bounces and permanent failures, and it identifies catch-all addresses that appear valid but don’t actually receive mail. This clarity is essential during transitions like removing rsa-sha1, where false positives can skew your results.

How MailTester supports secure, deliverable email infrastructure

You can update your DNS DKIM record to remove rsa-sha1 dependency by verifying your current setup, generating a new DKIM key with a modern algorithm like rsa-sha256, and ensuring your DNS records reflect it correctly. MailTester helps by testing whether your domains and addresses are deliverable, catching issues like invalid or role-based emails before they hurt your sender reputation. It also confirms inbox placement in real user environments, reducing the risk of your emails landing in spam.

Real-time email validation with actionable results

  • Use MailTester’s real-time verification API to check individual addresses or lists for validity, catching invalid, disposable, and role-based emails before delivery.
  • Each check returns a verdict—valid, invalid, catch-all, or risky—so you know exactly what’s safe to send.
  • MailTester flags role accounts (like admin@, support@) automatically, which helps avoid reputational damage from high bounce rates.

Inbox placement testing and AI-assisted reporting

  • Run inbox placement tests with MailTester’s inbox tester to see if your messages land in real inboxes across major providers, not just spam filters.
  • Each test uses actual mailbox environments, not simulations, giving you a realistic read on deliverability.
  • Use the in-app AI assistant to interpret complex deliverability reports and get plain-language recommendations for fixing issues like poor authentication or high bounce rates.
  • Integrate directly with SendGrid, Mailchimp, HubSpot, and Klaviyo to verify lists automatically during onboarding or campaign workflows.

When configuring DKIM, using only modern algorithms like rsa-sha256 is now an industry standard. The IETF’s guidance on DNS-based Authentication encourages moving away from legacy algorithms like rsa-sha1 due to known weaknesses. MailTester’s verification process helps you align with these standards by ensuring your email infrastructure isn’t compromised by outdated settings.

Best practices for post-DKIM update monitoring

After updating your DNS DKIM record to remove rsa-sha1, you must monitor deliverability, alignment, and DMARC compliance closely. Use tools to track inbox placement, verify your ESP’s authentication logs, and review DMARC reports for misalignment or failures. This ensures your emails continue to arrive in inboxes without being flagged or blocked.

Immediate post-update checks

  • Use a deliverability monitoring tool like Return Path’s deliverability guidelines to track inbox placement over the first 72 hours; aim for stable or improving placement rates.
  • Check your ESP’s authentication logs (e.g., SendGrid, Mailchimp, Amazon SES) to confirm DKIM signatures are now using a supported algorithm—like rsa-sha256—and that alignment with SPF and the email header domain is preserved.
  • Monitor your DMARC reports via a service like dmarc.org or your email provider’s reporting dashboard to detect any new failures, especially alignment issues between the from domain and DKIM or SPF.

Ongoing health checks

  • Review spam trap hits in your inbox placement reports—sudden spikes may signal misconfiguration or re-engagement issues due to a failed update.
  • Track engagement metrics (open rates, click rates, unsubscribe) over 1–2 weeks; sudden drops may indicate that your messages are being deprioritized or moved to folders.
  • Run a real-time inbox placement test using a tool like MailTester’s Inbox Placement Test to verify your email is landing in inboxes across Gmail, Outlook, and Yahoo with a clean score.
  • Verify your updated DKIM record remains active and correctly published by using a DNS lookup tool such as MxToolbox—especially if you manage multiple domains.

Let’s be clear: no single tool or check is enough. You need both automated monitoring and a human review loop. The key is consistency. Even if everything looks fine on day one, issues can emerge as receivers update their filtering behavior. Stay vigilant.

Final checks before calling the update complete

You’ve updated your DKIM record to remove rsa-sha1, but the change isn’t complete until you verify all systems are using the new key, SPF and DMARC remain intact, and no email workflows are still at risk. A single leftover system signing with rsa-sha1 can break authentication and hurt deliverability. Use MailTester’s inbox placement tool to test real-world delivery before moving on.

Verify your signing infrastructure

  • Check all email platforms (SendGrid, Mailchimp, HubSpot, etc.) to confirm they’re now using the new DKIM key. An outdated key still in use will trigger authentication failures.
  • Review any in-house email systems, CRM integrations, or automated tools that might be adding DKIM signatures. They may not have refreshed their keys after the update.
  • Use a real-time verification API to test a sample of sending addresses — ensure they pass with the new key and don’t return "invalid key" or "signature mismatch" errors.

Validate policy alignment and delivery health

  • Ensure your SPF record still includes all current authorized sending IPs. A mismatched SPF can cause authentication failures even with correct DKIM.
  • Confirm DMARC policy (p=none, p=quarantine, p=reject) is still active and aligned with your inbound email strategy. DMARC only protects you when all three (SPF, DKIM, DMARC) are properly configured.
  • Run a delivery test using a real inbox placement tool like MailTester’s inbox placement tester to verify your emails are landing in inboxes, not spam folders.
  • Check your sender reputation via third-party tools like Spamhaus or MxToolbox to confirm you're not listed on any blocklists.
A 2023 industry review by Return Path found that 37% of outbound emails still failed authentication due to misconfigured DKIM or SPF settings — even after key updates.

Summary: Remove rsa-sha1 from DKIM to future-proof deliverability

rsa-sha1 is no longer supported by major inbox providers. Using it risks delivery failure, even with valid emails.

Update your DKIM DNS record with a modern SHA-2 key. Keep the old key active during transition to avoid disruption.

Test the change with real email delivery tools. Use inbox placement testing and email verification to confirm reliability.

Sources

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

Frequently asked questions

What happens if I keep using rsa-sha1 in DKIM?

Mailbox providers like Gmail and Yahoo will reject or flag your messages, damaging sender reputation and reducing inbox placement.

Can I use DKIM without RSA keys?

Yes — some systems support ECDSA or EdDSA. However, RSA with SHA-256 is still the most widely supported and recommended.

How long does it take for DKIM updates to take effect?

DNS propagation typically takes 5 to 15 minutes, but full email delivery validation may take 24 to 72 hours.

Do I need to update all DKIM selectors?

Only update selectors that still use rsa-sha1. Retain active ones for continuity during migration.

Can I test DKIM changes without sending real emails?

Yes — use DNS lookup tools and email authentication testers to verify the record before sending.

What should I check if my emails still bounce after updating DKIM?

Verify SPF alignment, DMARC policy, and ensure the sending server is still using the correct key.

How does email verification help after DKIM changes?

It detects invalid, disposable, and role-based addresses that could trigger spam flags or bounces after configuration changes.

Is there a way to automate DKIM record validation?

Yes — tools like MailTester offer API-based validation, enabling automated checks during deployment workflows.

What’s the difference between DKIM and DMARC?

DKIM validates message integrity via cryptographic signatures; DMARC defines what to do when DKIM or SPF fails.

Does changing DKIM affect my sender reputation?

Yes — incorrect or outdated DKIM can harm reputation. Proper migration with testing preserves it.

Can MailTester verify DKIM configuration?

MailTester doesn’t validate DKIM directly, but it tests inbox placement and verifies sending addresses to support a healthy delivery stack.

Are there any tools to detect outdated DKIM algorithms?

Yes — MxToolbox, Mail-tester, and DMARC analyzers can flag rsa-sha1; use them before deployment.