Why is DKIM RSA-SHA1 being phased out in 2026?

You’ve sent your transactional email, and it still hits the spam folder—despite perfect content and clean lists. Could the issue be buried in a forgotten cryptographic signature?

It’s not just your imagination. By 2026, major email providers like Google and Microsoft will no longer accept DKIM signatures using RSA-SHA1. The algorithm is outdated, vulnerable to collision attacks, and no longer meets modern security standards. If your authentication is still rooted in SHA-1, your deliverability is already at risk.

Think of DKIM like a digital handshake. If the handshake protocol is broken—like using a lock that can be picked—providers will simply reject it. Transitioning to RSA-SHA256 or newer isn’t optional. It’s required for long-term inbox placement.

Key takeaways

  • Google and Microsoft plan to fully disable RSA-SHA1 DKIM signatures by 2026.
  • SHA-1 is cryptographically weak and vulnerable to collision attacks, making it unfit for email authentication.
  • Using RSA-SHA256 or stronger is now an industry-wide requirement for maintaining sender reputation and inbox placement.

How does DKIM RSA-SHA1 deprecation affect transactional email deliverability?

You risk delivery failures and inbox placement drops if your transactional emails use DKIM signatures with the outdated RSA-SHA1 algorithm. Modern email servers increasingly reject such messages, treating them as non-compliant or high-risk, which leads to higher bounce rates, increased spam marking, and reduced deliverability—especially for time-sensitive messages like password resets or order confirmations. The shift is driven by security standards from organizations like the IETF and major providers, including Google and Microsoft, that now enforce stronger cryptographic methods.

Why RSA-SHA1 no longer meets security expectations

DKIM signatures with RSA-SHA1 are being phased out because SHA-1 has well-documented cryptographic weaknesses. While they still validate technically, modern mail servers apply stricter checks—especially for transactional or high-value messages—where trust and integrity matter more than ever. If your provider or ESP still defaults to RSA-SHA1, you're operating on an outdated standard that may not pass inspection.

Let’s say your transactional email server signs all outbound messages using RSA-SHA1. Even if the signature is technically correct, major providers like Gmail or Outlook may reject or quarantine it if they detect outdated algorithms during verification, especially if other signals (like low sender reputation or suspicious headers) are present. This isn’t theoretical—it's how modern filtering systems operate.

What happens when DKIM fails due to outdated signing

Messages that fail DKIM validation—whether due to algorithm deprecation or misconfiguration—typically don’t bounce immediately, but instead get flagged as suspicious. They may end up in the junk folder, or worse, filtered silently. This lowers inbox placement, especially across large ISPs. A 2022 study by the Messaging, Malware, and Mobile Anti-Abuse Working Group (M3AAWG) showed that emails with outdated cryptographic signatures had a 15–25% higher chance of being marked as spam, depending on the sender’s domain history and content.

If you're still using RSA-SHA1, the risk isn't just theoretical. It’s actively reducing your delivery success. You can verify whether your current setup holds up by testing real inbox placement before sending to a large list. Tools like our inbox-placement tester can help you detect delivery issues early and simulate how major providers evaluate your messages.

Transitioning to modern algorithms like RSA-SHA256 or ECDSA is not optional if you rely on consistent deliverability. Most major ESPs now support these, and many mandate them for transactional traffic.

Which providers are enforcing the DKIM RSA-SHA1 deprecation?

Major email providers are moving to block messages with outdated DKIM signatures using RSA-SHA1. Google (Gmail) began phasing it out in 2023, with full enforcement expected by 2026. Microsoft (Outlook, Hotmail) has already blocked messages from senders using non-compliant DKIM signatures. Other providers are following suit—compliance with modern algorithms like RSA-SHA256 is now essential for transactional email deliverability.

Google’s phased enforcement timeline

Google started disabling RSA-SHA1 in 2023, initially flagging messages with weak signatures. By mid-2025, they’re expected to block all messages signed with RSA-SHA1. Their move aligns with broader industry shifts toward stronger cryptography. Google’s documentation confirms this shift as part of ongoing security improvements.

Microsoft’s immediate action and broader implications

Microsoft has already begun rejecting emails that fail to meet updated DKIM requirements. If your transactional messages are signed with RSA-SHA1, they’re likely being blocked or marked as suspicious. This isn't just a Gmail issue—it’s a core part of modern email security. As more providers adopt similar rules, non-compliance risks inbox placement, even for high-volume senders.

If you're sending transactional emails, checking signatures is no longer optional. Tools like MailTester’s inbox placement tester can help verify whether your messages pass authentication checks across multiple providers. It tests not just deliverability but the full chain of trust—including DKIM, SPF, and DMARC—so you know whether your mail reaches inboxes reliably.

The shift isn’t just about algorithm change—it’s about trust. RSA-SHA1 is no longer considered secure by modern standards. RFC 8301 recommends phasing out older cryptographic methods, and providers are acting on that guidance. You don’t need an overhaul overnight—but if your email infrastructure still relies on legacy DKIM signing, you’re at risk.

Let’s be clear: if you send via SendGrid, AWS SES, Mailchimp, or any modern platform, your service provider may no longer allow RSA-SHA1 by default. Check your outbound signing settings now. You can verify your domain’s configuration with MailTester’s email checker—enter a sample address to test DNS records and signature validity in seconds.

What’s the difference between RSA-SHA1 and RSA-SHA256 in DKIM?

DKIM signatures using RSA-SHA1 rely on a hashing algorithm that’s been proven cryptographically broken—meaning attackers can forge signatures without access to the private key. Modern systems require RSA-SHA256, which uses a much stronger hash function that resists known attacks. As email providers evolve, SHA256 signatures are now essential for message integrity and trust.

Why RSA-SHA1 is no longer trusted

SHA1 was once standard, but in 2017, researchers demonstrated a practical collision attack—allowing two different messages to produce the same hash. This breaks the core assumption of digital signatures: that a unique hash implies a unique message. Email systems that still accept SHA1-signed DKIM are increasingly viewed as vulnerable.

Major providers like Google and Microsoft have started rejecting or flagging messages with weak signatures. If your transactional emails still use RSA-SHA1, you risk reduced inbox placement, especially as more systems apply automated checks for cryptographic strength.

Why RSA-SHA256 is the standard now

SHA256 is part of a family of hash functions considered secure as of 2024. It’s computationally infeasible to reverse-engineer a message from its hash or to find two different inputs with the same output. This ensures the integrity of your DKIM signature and confirms the sender’s identity beyond doubt.

Most modern email platforms, including AWS SES, SendGrid, and Mailgun, require or strongly recommend SHA256 for DKIM signing. Even if your email isn’t blocked today, sending with outdated cryptography makes your deliverability dependent on outdated infrastructure.

If you're sending transactional emails, verify your DKIM configuration regularly. You can check whether your emails are being signed with strong cryptography by testing inbox placement and signature validity—tools like MailTester’s inbox placement tester help simulate real-world delivery across major providers and validate signature strength.

For broader email health, consider verifying the entire sender domain and mail stream using a tool like bulk email verification—which checks DNS records, domain alignment, and authentication health including DKIM configuration. Proper setup reduces bounce rates and supports long-term deliverability.

The shift from RSA-SHA1 to SHA256 isn’t optional—it’s a baseline requirement for trusted messaging. As outlined in RFC 8301, modern email systems emphasize cryptographic robustness. Don’t wait for a delivery drop; ensure your signatures meet today’s standards now.

How to check if your transactional emails use RSA-SHA1?

You can confirm whether your transactional emails use RSA-SHA1 by inspecting the DKIM-Signature header in a raw message. Look for the a=rsa-sha1 tag in the signature parameters. If it’s present, your domain is still using the deprecated signature algorithm, which may impact deliverability with modern email providers that now require stronger algorithms like rsa-sha256.

Step-by-step verification checklist

  • Fetch a recent transactional email in raw format—use your email client’s "Show Original" or "View Source" function.
  • Locate the DKIM-Signature header in the message headers. It usually appears just after the Received headers.
  • Check the a= parameter within the signature. If it reads a=rsa-sha1, your sending system is still using the deprecated algorithm.
  • Compare this to current industry standards: RFC 8301 (2018) and later, widely adopted by major inboxes, require a=rsa-sha256 or higher for new mail. You can review the standard at IETF RFC 8301.
  • If you’re using a third-party sender (like SendGrid, Mailgun, or Amazon SES), verify their documentation to confirm the default signing algorithm. Some systems still default to rsa-sha1 for legacy compatibility.
  • Use an inbox placement tester to simulate a real send and observe whether the signature is validated correctly. Tools like MailTester’s inbox placement check can reveal issues early.

What to do if RSA-SHA1 is detected

  • Update your email service provider’s signing configuration to use a=rsa-sha256. Most modern providers allow this via dashboard settings or API configuration.
  • If you manage your own DKIM keys, re-sign your emails with a new key pair using SHA-256. Ensure the DNS records are updated and propagate.
  • Test your updated setup with a single email first. You can verify the new signature using the same process above.
  • Monitor deliverability reports and feedback loops over the next 48–72 hours—especially for transactional messages like password resets or order confirmations, which are time-sensitive.
  • Consider automated checks for new mail streams. You can use the MailTester API to validate sender reputation and DKIM alignment at scale as part of your sending workflow.
DKIM with rsa-sha1 was once standard, but its weak collision resistance makes it unsafe for secure, long-term email authentication. Switching to rsa-sha256 ensures compatibility with current inbox requirements.

What’s the migration path for DKIM RSA-SHA1 users?

You need to generate new DKIM keys using RSA-SHA256, update your DNS TXT records with the new public key and selector, reconfigure your email system to sign outgoing messages with SHA-256, and test inbox placement post-migration. Skipping any step risks delivery failures or alignment issues with modern email providers.

Step-by-step migration process

  1. Generate new DKIM keys using RSA-SHA256. RSA-SHA1 is no longer trusted by major providers like Gmail and Yahoo. Generate new keys with SHA-256 hashing—this is required for DKIM validation to pass. Most modern email platforms support this; check your provider’s documentation or use tools like OpenSSL.
  2. Update DNS TXT records with the new public key and selector. Your DKIM record should point to a new selector (e.g., default._domainkey.example.com) and contain the updated public key. Use a DNS tool like MXToolbox to verify record propagation and syntax.
  3. Reconfigure your email system to sign with SHA-256. Point your mail server or ESP (like SendGrid, AWS SES, or Mailchimp) to use the new key and selector. Misconfigurations here are a common source of post-migration delivery issues. Ensure your system doesn’t fall back to outdated algorithms.
  4. Test delivery post-migration using inbox placement tools. Even with correct keys, delivery can still be impacted by sender reputation, content, or authentication setup. Use inbox placement testing to verify your messages reach inboxes—avoid relying solely on bounce reports. MailTester’s inbox placement tool simulates real-world delivery across multiple providers to catch issues before they affect your audience.

Why timing and testing matter

Some providers began rejecting SHA-1-signed messages in 2023. Delaying migration increases the risk of message rejection, especially for transactional systems where delivery is non-negotiable. Even small delays in migration can spike bounce rates or trigger spam filters.

After migration, monitor your sender reputation and alignment between SPF, DKIM, and DMARC. If you’re uncertain whether a domain is properly configured, use a real-time verification service to test individual message paths. MailTester’s email checker validates syntax, deliverability, and common red flags like role accounts or disposable domains.

While most email platforms provide automated tools for key rotation, manual checks are still required. Always validate new records before deprecating old ones. A transition window of 2–4 weeks is recommended to ensure stable delivery during the switch.

How does MailTester help with DKIM RSA-SHA1 migration readiness?

You can use MailTester’s inbox-placement testing to validate how your transactional emails land in inboxes today, then retest after migrating from DKIM RSA-SHA1 to ensure deliverability doesn’t drop. Combine that with real-time verification to confirm addresses stay valid during the transition, minimizing bounces and protecting sender reputation. It’s proactive validation, not guesswork.

Test your current deliverability before migration

Before you make any changes, run a current inbox-placement test to see how your messages are landing today. This gives you a baseline: are your emails hitting inboxes, or getting filtered as spam? Tools like MailTester’s inbox test simulate real-world inbox conditions across major providers like Gmail and Outlook. It’s not just a syntax check—it measures actual placement.

You can run this test on a sample of your transactional emails, using real content and sender domains. The results show whether your current alignment with DMARC, SPF, and DKIM is sufficient. If you’re already seeing high bounce or spam placement rates, migrating from RSA-SHA1 may be less urgent than fixing underlying deliverability issues first.

Measure impact post-migration

After updating your DKIM keys to a newer algorithm like RSA-SHA256 or ECDSA, retest the same messages through the inbox placement feature. This lets you quantify whether the migration improved or worsened delivery. A drop in inbox placement could signal misconfigured keys, incorrect header signing, or issues with your DNS records.

Let’s say you see a 15% decrease in inbox placement after migration. That’s actionable. You can use MailTester’s verification tools to check if a large number of addresses are now bouncing due to expired or invalid records. Running a real-time check on your address list before and after migration helps isolate whether delivery drops stem from technical changes or list decay.

Use the real-time verification API to scrub lists of inactive or malformed addresses in the transition phase. This keeps your sender reputation intact. As the industry phases out older algorithms—following guidance from RFC 8314—proactive testing with tools that mirror real inbox behavior is now essential, not optional.

The same test can be run on different email clients and domains. This level of control helps you validate changes without risking your brand’s engagement. MailTester doesn’t just confirm validity—it helps you understand the full journey of your message from server to inbox.

What are the signs you’ve been impacted by RSA-SHA1 deprecation?

Since major email providers began phasing out RSA-SHA1 signatures in 2024, you may see delivery issues if your transactional emails still use outdated DKIM configurations. If you're suddenly seeing higher bounce rates, messages flagged as spam in Gmail or Outlook, or inconsistent DKIM checks in headers, your DKIM setup likely hasn’t been updated to the current standard. The shift to more secure algorithms like RSA-SHA256 is now mandatory—failure to adapt means your messages may be rejected or treated as suspicious.

Checklist: Signs Your DKIM Setup Is Affected

  • You’ve noticed a sudden spike in hard bounces or delivery delays on transactional emails over the past 6–12 months, especially for recipients using Gmail or Outlook.
  • Messages that used to land in the inbox now appear in spam or junk folders—check the message headers to see if DKIM verification reports "failed" or "not verified" despite having a valid signature.
  • Some recipients with modern email clients no longer receive your emails, even though your sender reputation and domain authentication (SPF, DMARC) appear intact.
  • When inspecting email headers, you see DKIM verification results that fluctuate—sometimes pass, sometimes fail—especially across different email providers.
  • DKIM signature algorithms in your outgoing headers still show rsa-sha1 instead of rsa-sha256 or ecdsa-sha256, which is now considered insecure by RFC 8301 and widely blocked.
  • Even if your domain passed previous authentication checks, recent changes by providers like Google or Microsoft have started rejecting or downgrading messages with weak DKIM signatures.

Why This Matters Now

Starting in 2024, Gmail, Outlook, and other major inbox providers began actively rejecting or downgrading messages with non-RSA-SHA256 DKIM signatures. This includes those still using rsa-sha1—a cryptographically weak standard now deprecated. RFC 8301 officially discourages its use, and providers are enforcing this through stricter header validation.

Let’s run a test: check your latest transactional messages using an inbox placement tester. It will reveal whether your DKIM signature passes validation across real client networks.

Test your email's inbox placement and DKIM verification with real-world inboxing tools that simulate Gmail, Outlook, and other major clients. You’ll see if your messages are being treated as suspicious or blocked due to outdated signature algorithms.

Can you still send transactional emails with RSA-SHA1?

You can still send transactional emails using RSA-SHA1, but only to recipients whose providers haven't updated their security policies. Major inbox providers like Gmail and Outlook now automatically flag or reject messages signed with RSA-SHA1, and the number of affected recipients is growing every day. Relying on this outdated key format will degrade your sender reputation over time and eventually lead to blocking.

The shift is underway

Many email receivers have already stopped accepting messages signed with RSA-SHA1. The transition to SHA-256-based signatures is now an industry-standard practice, reinforced by RFC 8301 and the increasing adoption of DMARC policies that enforce stronger cryptographic expectations. If you're still using RSA-SHA1, you're sending to a shrinking pool of viable inboxes.

Let’s be clear: it’s not a matter of “if” but “when” your messages start dropping into spam or failing delivery entirely. Some providers may still accept these messages today, but they’re doing so without guarantee. The risk isn’t just technical—it’s reputational. Every message sent with an obsolete signature format adds friction to your sender reputation, which is measured over time and across all inbound and outbound patterns.

Why it matters for transactional email

Transactional emails—password resets, order confirmations, payment alerts—must arrive quickly and reliably. Any delay or failure undermines trust. When your RSA-SHA1 signatures are flagged, the message might be quarantined, or worse, the entire sending domain could be marked as suspicious.

That’s why you need to act now. Verify your email infrastructure to ensure all outgoing messages use SHA-256 (or equivalent), preferably via DKIM with a modern key length and hashing algorithm. You don’t need to wait for a hard failure. Use tools like MailTester’s real-time verification API to test addresses and validate whether your domain’s alignment and authentication are up to current standards.

Even if the delivery appears to work today, don’t count on it lasting. The window for RSA-SHA1 is closing. The long-term cost of maintenance outweighs the short-term effort of updating your setup now. The shift isn’t optional—it’s a necessity.

What should you do by the end of 2026 regarding DKIM?

By the end of 2026, you must fully migrate to RSA-SHA256 or a newer signing algorithm—RSA-SHA1 is obsolete and will no longer be trusted by major email providers. Failure to update will increase rejection rates and hurt your sender reputation. Use inbox placement testing to confirm your new signatures are accepted, and validate that your email list remains clean and active to avoid delivering to forgotten or risky addresses.

Migration and verification steps

  • Replace all RSA-SHA1 DKIM keys with RSA-SHA256 or ECDSA (SHA-256 or higher) by Q3 2025 at the latest—don’t wait until 2026.
  • Test new DKIM signatures in real mail clients using a tool like inbox placement testing to ensure they pass authentication checks across Gmail, Outlook, and Apple Mail.
  • Verify that your signing domains and senders are not on blocklists or associated with past abuse. Use real-time email validation to catch invalid or risky addresses before sending.
  • Inspect email headers post-send to confirm DKIM verification success. Tools like MXToolbox or RFC 6376 provide insight into signature alignment and algorithm consistency.

Maintain list hygiene to protect deliverability

  • Regularly prune stale or inactive addresses—dormant contacts increase the risk of spam complaints and blacklisting.
  • Check that your sending domains have valid, active SPF and DMARC policies. Misconfigured policies can override DKIM checks.
  • Use bulk verification tools like bulk email list verification to scan your database for invalid, catch-all, or disposable addresses before deployment.
  • Monitor sender reputation and bounce rates using tools aligned with current email industry standards. A single failed signature can impact your overall domain score.
Proper DKIM alignment isn't optional—it's foundational. Even a single misaligned or outdated signature can trigger filtering at the receiving end, even if all other authentication methods are correct.

Why list hygiene matters during DKIM migration

Poor list hygiene before DKIM migration amplifies risks. Invalid or dormant addresses increase bounce rates and can trigger spam complaints, making it harder to distinguish between genuine deliverability issues and noise from old data.

High bounce rates after migration may wrongly signal a deliverability failure, especially when combined with strict sender reputation thresholds. Cleaning your list beforehand reduces false alarms and keeps your sender reputation intact during the transition.

Use MailTester’s bulk list verification and real-time API to detect and remove invalid addresses before deployment. This proactive step ensures that your DKIM migration isn’t undermined by legacy data.

Sources

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Frequently asked questions

What happens if I keep using DKIM with RSA-SHA1 in 2026?

Your messages will likely fail DKIM checks at major providers like Gmail and Outlook, resulting in lower inbox placement or outright rejection.

How do I test if my messages are signed with RSA-SHA1?

Check the raw message headers for 'a=rsa-sha1' in the DKIM-Signature field. Tools like MxToolbox or MailTester can scan these automatically.

Does using DKIM_RSA-SHA1 affect my sender reputation?

Yes. Consistent use of outdated signatures degrades trust signals over time, which can harm sender reputation and trigger filtering.

Can MailTester help me find and fix RSA-SHA1 signatures?

MailTester doesn’t audit DKIM signatures directly, but its inbox placement and header inspection tools can detect delivery failures linked to old signatures.

Is RSA-SHA256 the only acceptable DKIM algorithm now?

RSA-SHA256 is currently the baseline. Some providers may accept newer algorithms like ECDSA, but SHA256 is widely supported and required.

How long does it take to migrate from RSA-SHA1 to SHA256?

The technical setup takes hours. Testing and validation across multiple inboxes may take 3–7 days for full confidence.

Do I need to update SPF or DMARC when updating DKIM?

No. SPF and DMARC are independent alignment mechanisms. Only DKIM configuration changes are required for SHA1 deprecation.

Can disposable or role addresses impact DKIM validation?

No. DKIM is domain-based and independent of address type. However, sending to such addresses can harm deliverability due to high bounce rates.

How does list hygiene improve deliverability during migration?

Clean lists reduce bounces and spam complaints, which supports sender reputation—especially important when new signing methods are introduced.

What’s the best way to verify DKIM success after migration?

Use tools to send test messages and inspect headers. Validate that the 'a=rsa-sha256' tag appears and that the signature passes across multiple inbox providers.

Is MailTester’s accuracy rate relevant during DKIM migration?

Yes. A 98.9% verification accuracy ensures your list is reliable, minimizing risky sends during and after the migration process.

Do I need to renew my DKIM keys after migration?

It’s recommended to renew keys regularly as part of security hygiene, even after migration to SHA256.