Why Is Your Email Getting Blocked Because of RSA-SHA1 DKIM?

You sent a perfectly crafted email. Your DKIM signature checks out. The message went to queue. Then—no delivery. No bounce. Just silence. And you’re left wondering: why?

It’s not your content. Not your sender reputation. Not even a misconfigured server. The issue might be buried in the cryptographic handshake your email uses: RSA-SHA1 in DKIM. This outdated signature method is now actively blocked or flagged by Gmail, Outlook, and Apple Mail.

Even if your DKIM is technically valid—because the keys match and the signature format is correct—it can still be rejected simply because RSA-SHA1 is no longer considered secure. Think of it like using a lock that still works, but the industry has moved on to stronger models. The door opens, but the guard won’t let you through.

Key takeaways

  • Major providers like Gmail and Outlook now block or flag emails using RSA-SHA1 DKIM due to outdated cryptographic standards.
  • DKIM validation is not a complete pass—algorithm choice matters just as much as technical correctness.
  • Even with valid DKIM, using RSA-SHA1 can lead to inbox placement failure or outright rejection, regardless of sender reputation.

How DKIM Works and Why Algorithm Choice Matters

Some email providers block messages with RSA-SHA1 DKIM signatures because RSA-SHA1 is a deprecated cryptographic algorithm. It’s no longer considered secure by modern standards. Providers reject such messages to prevent abuse, reduce forgery risks, and enforce stronger authentication. This isn’t about perfection—it’s about minimizing known weaknesses in systems that can be exploited.

DKIM’s Role in Email Authentication

DKIM adds a digital signature to every outgoing email. This signature proves the message came from your domain and hasn’t been altered in transit. The private key, stored securely on your mail server, signs the email. The public key, published in your domain’s DNS records, allows receiving providers to verify it.

When an email arrives, providers check the DKIM signature. If validation fails—due to a mismatched key, expired signature, or an outdated algorithm like RSA-SHA1—the email may be flagged or blocked. Even if the message technically reaches the inbox, it often lands in spam or is filtered out entirely.

Why Algorithms Matter Beyond the Basics

The algorithm used to generate the signature determines how hard it is to forge. RSA-SHA1, once standard, is now vulnerable to collision attacks. Major players like Google and Microsoft have formally deprecated it. The Internet Engineering Task Force (IETF) acknowledges this in RFC 8314, which outlines best practices for email authentication and recommends stronger alternatives.

Modern standards favor RSA-SHA256 or ECDSA. These offer far greater computational resistance to attacks. While older systems may still accept RSA-SHA1, relying on it today increases your risk of being blocked. It’s not just about compliance—it’s about ensuring your domain remains trusted.

Let’s be clear: no email provider wants to reject valid messages. But when an algorithm is known to be weak, they can’t afford the risk. Your domain’s reputation suffers if your emails get rejected due to outdated crypto.

You can test how well your domain’s authentication setup holds up. Use the inbox placement tester to simulate real-world delivery and catch issues like deprecated DKIM algorithms before they hurt your deliverability.

Which Email Providers Reject RSA-SHA1 DKIM Signatures?

Major email providers like Gmail, Outlook, and Apple Mail increasingly block or downgrade emails using RSA-SHA1 DKIM signatures, especially when sender reputation is weak. They now require or strongly prefer modern cryptographic algorithms like SHA-256 or ECDSA due to the known vulnerabilities in SHA-1. This shift is part of a broader industry move to phase out outdated security standards.

Gmail’s Stance on RSA-SHA1 Signing

Gmail treats RSA-SHA1 DKIM signatures as a red flag, particularly when paired with low sender reputation or other risky signals. Even if the message technically passes validation, these emails often land in spam folders or are silently downgraded in priority. The system uses a mix of cryptographic health checks and behavioral signals — weak signatures alone may not block, but combined with poor sending practices, they become a decisive factor.

Outlook and Apple Mail: Aggressive Validation

Microsoft Outlook and Apple Mail apply stricter validation policies. Outlook typically flags messages with RSA-SHA1 when other risks are present — such as a mismatched from address, low domain engagement, or a history of spam complaints. Apple Mail, known for its security focus, enforces tighter standards. It checks not only DKIM presence but also algorithm strength, and may reject or degrade messages that use SHA-1 under any circumstances.

These changes align with industry guidelines. The Internet Engineering Task Force (IETF) has long flagged SHA-1 as insecure, and both RFC 8301 and RFC 8308 mandate stronger hashing algorithms for email authentication. As a result, using SHA-1 doesn’t just break standards — it signals poor email hygiene.

Let’s be clear: you’re not fixing a broken system by relying on RSA-SHA1. The shift is inevitable. Modern email gatekeepers don’t just reject weak signatures — they treat them as indicators of broader sender risk. The longer you stay on older algorithms, the harder it becomes to maintain inbox placement.

For teams still using legacy DKIM configurations, now is the time to audit your signing setup. You can verify if your domain’s keys and algorithms meet current benchmarks using our email checker — it validates not only syntax but also real-world deliverability signals like algorithm strength and domain reputation. Proactive testing helps you avoid surprises when your first bulk campaign gets silently dropped.

What Happens When RSA-SHA1 DKIM Is Detected?

When an email includes a DKIM signature using RSA-SHA1, many modern email providers treat it as a potential security risk. As a result, the message may be silently dropped without notification, delivered to spam with a diagnostic note like “invalid or weak signature,” or rejected outright with a Delivery Status Notification (DSN) indicating content rejection. Over time, repeated use of outdated algorithms can harm your domain’s sender reputation, even if the email technically passes validation.

Why Providers Flag RSA-SHA1

While RSA-SHA1 was once standard, it’s now considered cryptographically weak. Major providers like Google, Microsoft, and Apple have phased out support for it in favor of stronger algorithms like RSA-SHA256. You’re not alone if your current setup still uses it—many legacy systems do. But the risk of delivery failure or blacklisting is real and growing. According to the IETF’s RFC 8301, SHA-1 is no longer considered secure for digital signatures in new implementations, and modern email gateways increasingly enforce this.

Common Outcomes and Hidden Risks

Even if your message slips through, you might not know it’s being filtered. Providers like Gmail often send no hard bounce, so no immediate feedback tells you the message failed delivery. Instead, it ends up in spam folders with a vague diagnostic like “signature invalid” or “weak authentication.” If your domain consistently sends with weak signatures, providers may start treating your traffic as suspicious—this hurts long-term deliverability, even for valid messages.

When a rejection occurs, it’s often via a DSN with a code like 550 5.7.1, which stands for “Content rejected” and usually points to policy or cryptographic failure. These messages are logged, and repeated incidents can impact your domain’s reputation. Even if the email address is valid, a weak signature may prevent inbox placement entirely.

Let’s be clear: a single failure isn’t the end of the world, but repeated use of RSA-SHA1 across your mail stream increases the odds of being flagged as a risk. If you're sending from a domain with high-value traffic, this is not a minor technicality—it’s a deliverability issue with measurable impact.

Use tools that scan your email infrastructure for weak cryptographic signatures. Check individual addresses or verify your entire list with bulk list verification to identify outdated configurations before they cost you in inbox placement.

How to Check if Your DKIM Uses RSA-SHA1

You can check if your DKIM signature uses RSA-SHA1 by inspecting your domain’s DNS TXT record. Look for the a=rsa-sha1 parameter in the DKIM public key. If it’s present, your messages may be blocked by modern email providers. Use a header inspection tool to confirm if your emails are flagged during delivery.

  1. Fetch your domain’s DKIM TXT record using a tool like MxToolbox or the terminal command dig TXT yourdomain.com. This reveals the DKIM public key published in DNS.
  2. Look for the v=DKIM1; tag in the result. This confirms the record is a DKIM key. Then scan the parameters for a=rsa-sha1. Its presence means your signature algorithm is deprecated.
  3. Understand the risk. Major providers like Gmail, Yahoo, and Apple Mail have stopped accepting RSA-SHA1-signed messages since 2022. Servers that still allow them are considered insecure and may reject your message outright.
  4. Test live delivery signals using a tool like MailTester’s inbox placement test. This checks not just the header, but how your message behaves in real inboxes—flagging algorithm issues before they cause mass bounces.

Why This Matters for Deliverability

While RSA-SHA1 was once standard, it’s now a known vulnerability. The SHA-1 hash function is weak by modern standards and has been cracked in practice. Major email providers enforce strict policies around this, and ignoring them leads to low inbox placement or outright rejection.

What to Do Next

If your signature uses RSA-SHA1, update your DKIM keys to use a=rsa-sha256 or a=ed25519. This ensures compatibility with today’s security requirements. Many email platforms (SendGrid, Mailchimp, etc.) let you reconfigure signing via DNS. After changing, re-test with a header tool to confirm the new algorithm is active.

The shift is straightforward but essential. You don’t need to wait for problems to arise—checking your current setup takes under two minutes. Tools like MailTester’s single address checker can help confirm whether a specific recipient’s server will accept your message based on the signature type.

For teams managing large lists, regular scanning with bulk verification is a smart practice. It catches outdated or insecure signing before sending—protecting your sender reputation and inbox placement.

How to Fix: Upgrade to a Modern DKIM Signature Algorithm

Some email providers block messages with RSA-SHA1 DKIM signatures because they’re considered insecure and outdated. To fix this, replace RSA-SHA1 with RSA-SHA256 or ECDSA when generating your DKIM key pair. Update your email server or ESP to use the new key, publish the new public key in DNS, and ensure all outgoing mail is signed with the modern algorithm. Test delivery with inbox placement tools instead of relying only on bounce logs.

Step-by-Step: Transition to Modern DKIM Signatures

  1. Generate a new DKIM key pair using RSA-SHA256 or ECDSA. RSA-SHA1 is no longer trusted by major providers like Gmail and Outlook. Using RSA-SHA256 or ECDSA ensures your signature meets current security standards. These algorithms are widely supported and recommended by the IETF in RFC 8301 for secure email authentication.
  2. Update your email server or ESP to use the new key during signing. If you use an ESP like SendGrid or Amazon SES, configure it to sign outbound messages with the new key. Some platforms allow you to upload a public key and switch signing methods in their dashboard. For self-hosted setups, update your MTA (like Postfix or Exim) to use the new key during message signing.
  3. Publish the new public key in your DNS TXT record. The public key must be published under the correct selector and domain in DNS. Double-check the syntax—incorrect formatting breaks DKIM validation. Use tools like MXToolbox to verify the TXT record resolves correctly.
  4. Ensure all outgoing messages are signed with the updated algorithm. You may have legacy systems or third-party tools that still sign with RSA-SHA1. Audit your outbound flow to confirm there are no exceptions. Even one message with an old signature can trigger flagging, especially in high-volume campaigns.
  5. Test delivery after rollout using inbox placement tools. Bounce logs won’t catch delivery issues due to filtering. Instead, use inbox placement testers to see how your messages land in real user inboxes across Gmail, Yahoo, Outlook, and others. MailTester’s inbox placement tests simulate multiple user environments and identify delivery issues early—before they affect your sender reputation.

Why Testing Matters

Even with correct DKIM setup, some providers may still block messages if your sender reputation is low, or if alignment or SPF checks fail. Testing with real inbox simulations catches issues that bounce logs miss. It’s a reliable way to confirm delivery before sending to a large list. Always validate your configuration in a staging environment first.

Why Don't All Providers Block RSA-SHA1 Yet?

You might wonder why some email providers still accept messages with RSA-SHA1 DKIM signatures, even though they’re considered outdated. The short answer: there’s no universal rule mandating their rejection. Many providers, especially older or low-traffic systems, still accept RSA-SHA1 by default because upgrading DKIM requirements requires coordination, testing, and infrastructure changes — something not all systems prioritize or can afford right now.

Legacy Infrastructure Slows Adoption

Large enterprises and older email platforms often run on legacy systems that aren’t easily upgraded. These systems may still be configured to handle RSA-SHA1 without issue, and updating them can disrupt existing workflows. Until they’ve verified compatibility with newer algorithms like RSA-SHA256 or ECDSA, many operators choose to keep existing settings active. This creates pockets of acceptance that persist even as newer standards spread.

Providers Decide Independently

There’s no global mandate forcing all email providers to reject RSA-SHA1. Each provider sets its own policies based on internal risk assessments and security roadmaps. Some prioritize rapid adoption of modern cryptography, while others maintain backward compatibility for stability and deliverability. This variation means your message might reach one inbox but get blocked in another — even if everything else (SPF, DMARC, content) checks out.

For example, a 2023 report from the Internet Society noted that while most major email services have moved to SHA-2, some smaller or enterprise-specific systems lag behind due to configuration delays and testing cycles. The ongoing transition is managed case-by-case, not enforced at scale.

This inconsistency makes deliverability unpredictable. A perfectly signed message using RSA-SHA1 might land in one inbox but be filtered elsewhere. That’s why validating your sending infrastructure — including your DKIM configuration — is critical before a large send. Tools like MailTester’s email checker help you identify potential issues early, including weak or outdated cryptographic signatures, so you don’t get caught off guard by inconsistent filtering.

Can You Still Use RSA-SHA1 Today Without Issues?

You can still send messages signed with RSA-SHA1, but it’s no longer safe to rely on. While some major providers like Gmail and Outlook may still accept these signatures, especially from established domains with strong sender reputations, the risk of rejection or filtering is increasing. As industry standards evolve, systems are phasing out support for weak cryptographic practices, making RSA-SHA1 a short-term stopgap—not a future-proof solution.

The Risk Is Rising

Let's be clear: using RSA-SHA1 is akin to driving a car with a warning light on. You might get away with it today, but the longer you ignore it, the higher the chance of a breakdown. Providers such as Google and Microsoft have been actively deprecating older algorithms for years. According to RFC 8301, which updates standards for DKIM, SHA-1 is now considered cryptographically weak and should not be used for new deployments. This isn’t just policy—it’s security consensus, echoed in guides from the Internet Engineering Task Force and the National Institute of Standards and Technology (NIST).

Even if your domain has a long history and high reputation, don’t assume you’re immune. The moment you scale or on-board a new email provider (especially via third-party services), you’re exposed. Some ESPs and spam filters now flag or block mail with RSA-SHA1 signatures without exception—even if the rest of your stack is clean. If you're relying on this signature, you’re not just risking delivery—you're creating an avoidable weak point in your email security chain.

What Should You Do Instead?

Transitioning to modern algorithms like RSA-SHA256 or ECDSA is the only sustainable path. These are widely supported across all major email providers and align with current security best practices. It’s a technical update, not a radical overhaul. Most modern email platforms—SendGrid, Amazon SES, Mailchimp—support newer DKIM standards out of the box. If your current system doesn’t, it’s worth auditing your DNS and signing setup now, before providers begin automatic rejection.

Use tools to test your deliverability *before* you send. Our inbox placement test checks how your message lands across major providers, including how they handle older signatures. You’ll see real-world results—no guesswork. Run a deliverability test to validate your current setup and avoid surprises after a campaign.

Don’t bet on tolerance lasting. As more systems enforce compliance, the window for RSA-SHA1 will close. The only reliable strategy is to upgrade—not wait.

How MailTester Helps Verify Deliverability Risks Like This

You can’t rely on email providers silently rejecting messages just because they use outdated cryptography like RSA-SHA1 DKIM signatures. MailTester’s inbox placement tests send real messages through Gmail, Outlook, and Apple Mail, and they detect whether such signatures are flagged or blocked during delivery. This catches risks before they hurt your reputation, and our 98.9% accuracy ensures you’re not being misled by false alarms.

Real Messages, Real Inboxes, Real Signals

Let’s be clear: you can’t test inbox delivery with fake messages or theoretical checks. MailTester sends actual emails through real infrastructure—Gmail, Outlook, and Apple Mail—to see how they land. This includes checking if older cryptographic methods like RSA-SHA1 DKIM are rejected outright. Some providers now treat them as insecure, even if technically valid, so catching this early prevents delivery failure.

When you run an inbox placement test, you don’t just get a “delivered” or “bounced” result. You get signal data: whether the message was quarantined, tagged as spam, or outright blocked. If a provider rejects a message due to RSA-SHA1, MailTester flags it directly. That’s real feedback—not guesswork.

Proactive Risk Detection Across Your Stack

Our verification API doesn’t just check if an address exists. It analyzes domain-level signals, including outdated crypto practices. It assigns a risk score to each address based on known deliverability red flags—like weak authentication, outdated DKIM methods, or known spam behavior. This lets you weed out problematic emails before they ever leave your server.

For bulk senders, our bulk verification tools scan thousands of addresses at once, identifying those with poor domain hygiene, including those tied to known RSA-SHA1 signatures. You can filter these out before sending, reducing bounce rates and protecting your sender reputation.

The key advantage is accuracy. With 98.9% precision, you’re not sacrificing good addresses to avoid false positives. This level of reliability means you can act on the data confidently. For deeper insight, you can explore how cryptographic standards evolve—see the IETF’s guidance on outdated algorithms at IETF TLS documents, where deprecation of SHA-1 is well-documented.

Whether you’re validating a list, checking one address, or testing your full delivery chain, MailTester gives you the tools to find and fix issues like RSA-SHA1 before they cause problems. See how it works for yourself: test your inbox placement or verify your list at scale.

Some email providers block messages signed with RSA-SHA1 because it’s outdated, mathematically weak, and no longer considered secure. Modern systems prefer RSA-SHA256 or ECDSA for DKIM signatures. If your emails still use RSA-SHA1, you’re at risk of being filtered or rejected—especially by Gmail, Outlook, and corporate gateways. Let’s fix that.

Keep Your DKIM Setup Strong and Up to Date

  • Always generate new DKIM keys using RSA-SHA256 or ECDSA—never RSA-SHA1. These are the industry standard.
  • Review your existing DKIM configuration every quarter. Key strength can degrade if keys aren’t renewed or rotated regularly.
  • Check your DNS records for any DKIM public keys still using SHA1 or weak key sizes (e.g., 1024-bit RSA). Replace them immediately.
  • Use MailTester's bulk verification tool to audit sender domains and detect flawed or outdated signing configurations across your email list.

Monitor and Validate in Real Time

  • Enable and review authentication logs from your email service provider. These logs often flag weak signatures before delivery fails.
  • Test real-time delivery before launching campaigns. Use tools that simulate inbox placement across major providers.
  • Don’t rely only on hard bounce rates. A weak DKIM signature can cause delivery failure without a bounce—messages are silently dropped or quarantined.
  • Use MailTester’s inbox placement tester to validate how your messages land in real inboxes, including whether DKIM authentication is passing.
  • Refer to RFC 8301, which outlines modern DKIM security requirements, for authoritative guidance on acceptable signature algorithms.
Authentication is only as strong as your weakest key. A single outdated signature can damage your sender reputation across multiple platforms.

You don’t need to wait for a bounce to know something is wrong. Proactive checks—like verifying your DKIM setup with real-time delivery testing—catch issues before you send. And if you're using an older email service, ask if it supports SHA256 signing. If not, consider upgrading.

The Bottom Line: Outdated Algorithms Cost You Deliverability

RSA-SHA1 DKIM signatures are no longer accepted by major email providers for reliable inbox delivery. Even if your messages still arrive today, relying on this outdated algorithm increases the risk of future delivery failures.

Upgrading your DKIM algorithm—switching to a modern, secure option like SHA-256—is a small technical change with a significant impact on deliverability. It’s a proactive step that future-proofs your email program.

Don’t rely on basic syntax checks. Real inbox placement depends on how providers evaluate your sender reputation, authentication, and message content. Use tools that test real delivery behavior, not just compliance.

Sources

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Frequently asked questions

Does DKIM still work if it uses RSA-SHA1?

It may deliver, but modern providers increasingly reject or downgrade messages using RSA-SHA1. Relying on it is a risk to inbox placement.

Can I keep RSA-SHA1 if my email is still getting through?

It may work temporarily, but as security standards evolve, delivery reliability will decrease. It's not future-proof.

Which DKIM algorithm should I use in 2026?

Use RSA-SHA256 or ECDSA. These are current industry standards and widely supported by major email providers.

How do I know if my DKIM signature is weak?

Check your DNS TXT record for 'a=rsa-sha1'. If present, your signature uses an outdated algorithm. Use a tool like MailTester to verify delivery behavior.

Does MailTester check DKIM signing algorithms?

Yes. MailTester’s inbox placement tests detect weak DKIM algorithms like RSA-SHA1 during real delivery testing.

Do I need to change my DKIM key if I switch email providers?

Yes. Each provider generates and manages keys independently. If you move platforms, you must generate and publish a new key.

What happens if my email client uses RSA-SHA1?

Modern email providers may reject the message or flag it as low trust, reducing inbox placement and increasing spam detection risk.

Why did Microsoft decide to block RSA-SHA1 DKIM?

Microsoft follows industry standards to improve email security. SHA-1 is considered cryptographically weak and has been deprecated since 2011.

Can I use multiple DKIM keys for different servers?

Yes. You can have multiple DKIM selectors, each with different algorithms. Just ensure they’re properly published in DNS.

Is there a way to test DKIM signature strength without sending an email?

No. Real delivery testing is required to detect how providers evaluate your signature. Tools like MailTester simulate this with actual inbox placement.

How common is RSA-SHA1 use today?

It’s declining, but still found in older systems, legacy platforms, or poorly maintained email setups. Most new deployments avoid it.

Can I fix DKIM algorithm issues without technical help?

Yes, with clear steps. Most ESPs provide documentation for key rotation and algorithm update. MailTester helps confirm success post-change.