Fix DKIM a=rsa-sha1 Issues

Why Is Your DKIM Signature Using a=rsa-sha1 and Why It Matters

You sent an email. It was delivered. But your inbox placement is still low, and some clients are showing up in spam. You check your logs. The culprit? A single line in your DKIM signature: a=rsa-sha1.

This isn't just a technical footnote. It’s a red flag. Major providers like Google and Microsoft are actively rejecting emails using this outdated hashing algorithm—especially when authentication is weak. If your DKIM isn’t updated, your messages are at risk, even if they technically “send.”

Your sender reputation isn’t built on volume. It’s built on consistency, security, and compliance. Using a=rsa-sha1 undermines both technical and reputational trust—making it harder to land in inboxes, even with clean content.

Key takeaways

DKIM signatures with a=rsa-sha1 are being phased out by major email providers like Google and Microsoft, leading to higher rejection rates.
Even if your emails deliver, failing to update this signature harms sender reputation and increases likelihood of spam filtering.
Switching to a=rsa-sha256 or a=rsa-sha512 in your DKIM configuration ensures long-term deliverability and compliance with modern email security standards.

How to Check If Your DKIM Signature Uses a=rsa-sha1

You can check if your DKIM signature uses the deprecated a=rsa-sha1 algorithm by retrieving your domain’s DKIM TXT record via DNS tools like MxToolbox or the command-line dig. If the record contains a=rsa-sha1, your signature is outdated and may hurt deliverability. Run a real-time inbox placement test to see how your emails are received across providers like Gmail and Outlook.

Retrieve your DKIM DNS record using a tool like MxToolbox or the dig command. Enter your domain and look for a TXT record starting with selector._domainkey.yourdomain.com.
Inspect the record for the a= parameter. If the value is a=rsa-sha1, your DKIM signature uses a deprecated algorithm. This is no longer accepted by many major email providers, including Gmail and Microsoft, starting in 2024.
Verify the impact with real inbox placement testing. Use a service like MailTester’s inbox placement test to send sample messages to real inboxes across providers. This shows if your email lands in the inbox, spam folder, or is blocked entirely — including issues caused by outdated DKIM.

Why This Matters

DKIM verification is part of email authentication. If your signature uses a=rsa-sha1, it may fail due to weak hashing. Major gateways now require stronger algorithms like rsa-sha256 or ed25519 for modern email systems.

What to Do Next

Once confirmed, update your DKIM record with a new key using a=rsa-sha256. This is done via your email service provider's settings—commonly in providers like SendGrid, Mailgun, or your hosting platform. Always retest after changes.

Some providers still accept rsa-sha1 for legacy reasons, but it’s a growing risk. According to industry best practices, including those outlined in RFC 6376, newer algorithms are preferred and increasingly mandatory. Even if your email seems to send now, you risk long-term deliverability issues as providers phase out older standards.

Use a bulk verification tool like MailTester’s list validator to check your entire address list for delivery risks — including invalid or poorly authenticated addresses — before sending.

What Happens When You Use a=rsa-sha1 in 2025 and Beyond

You risk email rejection, reputational damage, and domain-level filtering. By 2025, most major providers flag or block messages using a=rsa-sha1 because it relies on a weak cryptographic hash. Even if your email arrives, the low trust signal harms inbox placement and future deliverability. Some abuse databases may record the failure, leading to broader filtering across networks.

Receiving servers increasingly reject or distrust a=rsa-sha1

Major email providers, including Gmail, Outlook, and Yahoo, now treat a=rsa-sha1 as obsolete. While they don’t always block outright, you’ll often see a=rsa-sha1 signatures marked as low trust or failing validation. The receiving server logs this as a security weakness, which can trigger automated filtering decisions even if the message itself is clean.

When your DKIM signature uses outdated crypto, it’s like showing up with a cracked key. Even if you’re allowed in, the door is left ajar for suspicion. The infrastructure behind email delivery has evolved — algorithms like rsa-sha256 are now standard, and relying on older methods sends a signal that your domain may bypass security best practices intentionally or unintentionally.

Long-term damage to sender reputation and deliverability

Even if your message gets delivered, the fact that it uses a deprecated signature reduces your overall sender reputation. ISPs track cryptographic integrity as part of their reputation models. A pattern of weak signatures over time can result in lower priority in inboxes, especially for bulk or transactional mail.

Some providers maintain records of known cryptographic flaws in their abuse tracking systems. If your domain is associated with a=rsa-sha1 in large volumes, it may be flagged in real-time databases such as those used by Spamhaus or MxToolbox. These flags don’t disappear — they linger, affecting all emails from your domain, even after you fix the issue.

Let’s be clear: fixing DKIM isn’t just a technical task. It’s a credibility upgrade. If you're sending from a domain that still uses rsa-sha1, you’re implicitly signaling that your mail flow isn’t being monitored or maintained to current standards. The risk isn’t just bounce rates — it’s long-term deliverability erosion.

Use DNS checks with tools like MxToolbox or DNSstuff to verify your DKIM records. If you're unsure whether your domain uses rsa-sha1, test it with a dedicated real-time email checker before sending. You can also verify your entire list with bulk verification to ensure all addresses conform to modern standards.

How to Update Your DKIM Signature to a Modern Algorithm

You can fix a deprecated a=rsa-sha1 DKIM signature by updating your sending platform’s configuration or regenerating your private key with a=rsa-sha256, then publishing the new public key in DNS. Confirm the change with a DNS lookup and test deliverability using a real-time inbox placement check. This ensures your emails authenticate properly and avoid being dropped by modern receivers.

Step-by-step: Migrate from rsa-sha1 to rsa-sha256

Confirm your sending platform supports rsa-sha256—contact your provider (SendGrid, Mailchimp, HubSpot, etc.) to verify they allow you to configure a modern DKIM algorithm. Many platforms still default to sha1 for backward compatibility, but it’s no longer trusted by major inboxes.
If you manage DKIM yourself, regenerate your private key using SHA-256 instead of SHA-1. The key generation process is platform- or tool-specific, but ensure the resulting signature uses a=rsa-sha256. This is a critical step—SHA-1 is cryptographically weak and rejected by receivers like Gmail and Outlook.
Update your DKIM DNS record with the new selector and public key. Use a DNS lookup tool like MXToolbox or RFC 6376 to confirm the record is published correctly and accessible. The new record must match the selector used in your email headers.
Test the new DKIM signature in context using an email delivery test tool. Send a test message from your domain and validate both the header and DNS record. Check inbox placement with a tool like real-time inbox testing—it checks not just DKIM, but spam scores, routing, and final delivery.

Why this matters today

Major platforms like Google and Microsoft now reject or flag emails with outdated cryptographic signatures. While a=rsa-sha1 still works in some legacy environments, it’s increasingly ignored or treated as suspicious. Using a=rsa-sha256 aligns with current authentication standards and improves inbox placement.

Even if your email sends, a failing DKIM signature can hurt your sender reputation over time. Once set, modern DKIM signing is stable—no need to reconfigure unless changing providers or keys. Always verify changes with a real delivery test; DNS alone doesn’t confirm inbox delivery.

For bulk sender operations, use an email verification tool like MailTester’s bulk verification to clean your list before sending. Validating email addresses early prevents issues with deliverability and reputational damage down the line.

Common Misconceptions About Fixing a=rsa-sha1 DKIM

You don’t need to overhaul your email system to fix a deprecated a=rsa-sha1 DKIM signature. Only the DKIM key and its DNS record require updating. SPF and DMARC remain unaffected, and fixing the algorithm doesn’t erase your email history — it just ensures future messages are properly authenticated. The change is targeted and low-risk when done correctly.

It’s Not a Full System Overhaul

Many think replacing a=rsa-sha1 demands rebuilding your entire outbound email setup. That’s not true. You’re only updating one component: the DKIM private key used to sign messages and the corresponding public key in DNS. Your sending infrastructure, authentication policies, and historical sending data stay intact.

Let’s be clear: this isn’t about changing your entire email provider or migrating servers. It’s about regenerating a digital signature that’s no longer considered secure by modern standards.

Why SPF and DMARC Won’t Help

SPF and DMARC don't address DKIM signature algorithms. SPF checks sender IP legitimacy, DMARC enforces alignment between SPF and DKIM results — but neither controls how emails are signed. If your DKIM uses a=rsa-sha1, updating SPF or DMARC won’t resolve it.

In fact, treating DKIM as the same type of policy is a common mistake. You can have DMARC set to enforcement but still fail if the DKIM signature algorithm is outdated — a situation many inbox providers now flag.

According to RFC 8301, a=rsa-sha1 is no longer recommended due to known cryptographic weaknesses. While some older email systems still accept it, leading inbox filters like Gmail and Microsoft’s Exchange are increasingly rejecting messages with weak signatures. IETF RFC 8301 formalizes the deprecation of SHA-1 in DKIM.

Once you update your DKIM key to use a stronger algorithm like a=rsa-sha256, your new messages will pass inbox filtering more reliably. It’s not about retroactively fixing old emails — it's about future-proofing your authentication.

Before you make the change, check your current DKIM setup using an email verification tool. MailTester’s inbox placement tester can help validate how your messages will be received, including DKIM alignment. You can also use our email checker to test individual addresses and confirm authentication readiness.

How MailTester Can Help You Detect and Prevent DKIM Signature Issues

You can catch deprecated DKIM algorithms like a=rsa-sha1 before they cause bounces or spam filtering by testing individual addresses and full message headers in real time. MailTester’s API and inbox-placement tests reveal authentication failures early, so you fix them before sending to large lists.

Test Before You Send

Use MailTester’s real-time verification API to analyze a single sender address, including full header checks that include DKIM signatures.
Send a test message through MailTester’s inbox-placement tester to see how it lands in Gmail, Outlook, Apple Mail, and other providers—caught issues include a=rsa-sha1 deprecation.
Verify your email setup across multiple domains and senders: a single signature flaw can break deliverability across all recipients.

Automate and Prevent Issues at Scale

Run bulk verification on your email list using MailTester’s email list verify tool to flag addresses with broken or outdated DKIM authentication.
Check for deprecated algorithms like a=rsa-sha1 in your outbound messages—this algorithm is no longer accepted by major inbox providers and leads to rejection or strict filtering.
Enable integrations with Mailchimp, HubSpot, Klaviyo, or SendGrid to automatically validate every address before it’s sent, reducing bounce rates and protecting your sender reputation.

The IETF’s RFC 8301 explicitly recommends against using rsa-sha1 due to cryptographic weaknesses. Gmail, Outlook, and other major providers are filtering or rejecting messages with such signatures as of 2023—no exceptions.

Let’s be clear: a single broken DKIM signature doesn’t just affect one email. It can signal poor sender hygiene to receivers, lowering trust across your entire domain. By detecting these issues before sending, you avoid both hard bounces and long-term reputation damage.

MailTester gives you the exact data you need—accurate, real-time, and backed by a 98.9% verification accuracy rate. You don’t want to guess what’s causing a delivery failure. You want to see it.

Best Practices to Avoid a=rsa-sha1 in Your Email Stack

Use modern DKIM algorithms like rsa-sha256 across all your email tools—marketing platforms, transactional services, and senders. Audit your DNS records every six months to catch drifting configurations. Automatically clean out invalid or broken email addresses, including those failing verification, to prevent DKIM issues. You don’t have to wait for bounces or spam complaints to act.

Verify & enforce modern DKIM globally

Check all third-party email services (like Mailchimp, Klaviyo, or SendGrid) to confirm they use a=rsa-sha256 or newer, not a=rsa-sha1. Many still default to deprecated settings.
Use your email platform's DKIM key management page to verify the algorithm in use—some tools allow you to override the default, so don’t assume it’s correct.
When integrating new tools, ask what algorithm they use by default; if it’s not modern, request they update or reject the integration until they do.

Proactively maintain inbox deliverability hygiene

Run a bulk email verification every six months—especially if your list is older than 12 months. This catches outdated or broken domains.
Use a real-time verification API to check individual addresses before sending. You can catch bad DKIM setups early, before they hurt sender reputation.
Automatically remove addresses flagged as invalid, catch-all, or risky from your sends. These often result in rejected messages or spam flags, especially when DKIM fails.

According to the IETF, sha1 is no longer cryptographically secure for digital signatures—it’s been deprecated in favor of stronger alternatives since 2018. Relying on a=rsa-sha1 undermines email integrity and increases the chance of your messages being blocked.

Fixing DKIM is not a one-time task. It’s part of routine sender hygiene, like checking for blacklists or validating SPF.

Tools like MailTester’s bulk email verification identify invalid addresses and detect issues like broken DKIM records before you send. Catching problems early reduces bounces, keeps your reputation intact, and prevents your messages from landing in spam folders.

What’s the Real Impact of Using a=rsa-sha1 on Sender Reputation?

A single rejected DKIM signature using the outdated a=rsa-sha1 algorithm can lower your sender reputation score—especially if it happens repeatedly. Since spam filters track long-term behavior, even older messages with broken authentication may trigger blocks if they resurface in inboxes. Reputation is shared across IPs and domains, so one weak signature can hurt all your outbound mail, not just that one message.

Reputation Is Cumulative and Shared Across Infrastructure

Let’s be clear: sender reputation isn't just about today’s send. It’s built over time and affects every email you send, regardless of when it was sent. A 2020 study by Return Path (now Validity) found that even old, low-volume mail with failed authentication still contributes to filtering decisions when it appears in recipient inboxes. That means a message from 2021 with a broken DKIM signature might still be a red flag to modern filters if it’s opened or forwarded. You might think, “Well, it’s just one email.” But if that one email fails DKIM due to a=rsa-sha1, and the same domain or IP sends thousands of others with the same issue, your reputation drops in a measurable way. According to RFC 6376, which defines DKIM, the a=rsa-sha1 tag specifies a hashing algorithm that’s now considered deprecated. Major providers like Google and Microsoft are actively discouraging its use in new setups.

Why Fixing it Isn’t Just a Technical Task

You’re not just fixing a signature—you’re protecting a long-term relationship with inbox providers. When a DKIM check fails, it’s logged as a failed authentication event. The more events you accumulate, the higher the chance your domain gets added to a reputation blacklist, even if just one of those events was due to outdated algorithms. If you’re using a third-party email platform or email marketing tool, verify whether it still defaults to rsa-sha1. Many older systems do. MailTester’s verification API can help you catch invalid or deprecated signatures in advance. Check individual addresses before they get sent using our [email checker](https://mailtester.com/email-checker/) to test deliverability and alignment. Even if your current system isn’t flagging the issue, a single failed verification can be the spark that triggers a broader reputation drop. Use tools that test real-world delivery conditions—not just syntax—and fix it before the next batch of campaigns goes out.

Why Legacy Systems Still Use a=rsa-sha1 Today

Many older email setups still use a=rsa-sha1 because they were built before SHA-256 became an industry standard. Even though major platforms like Google and Microsoft now reject messages with outdated signatures, some systems—especially in large enterprises or government agencies—haven’t updated their configurations, either due to inertia or lack of visibility into delivery problems.

Older Infrastructure Without Immediate Upgrades

Systems deployed before 2016 often default to rsa-sha1 because it was the baseline at the time. These setups weren’t designed with future algorithm upgrades in mind. When you're managing hundreds of mail servers with no dedicated email operations team, prioritizing security updates can fall behind routine maintenance.

Even when newer systems are introduced, they often mirror legacy settings to maintain consistency across fleets. If the rest of your environment uses rsa-sha1, a new server inherits the same policy—especially if the default configuration in your mail transfer agent (MTA) doesn’t allow easy changes.

Administrators May Be Unaware of the Risk

Many admins aren’t tracking email delivery failures tied to cryptographic validation. A bounce message indicating “signature verification failed” is often dismissed as a transient network glitch. Without tools that show the specific reason for rejection, it’s easy to miss that your DKIM signature uses deprecated cryptography.

For example, RFC 8314, published in 2018, explicitly recommends phasing out rsa-sha1 due to known cryptographic weaknesses. Yet, widespread adoption took years. Even today, many security audits don’t catch this unless specifically tested.

Let’s be honest: if your team is using a default MTA configuration from 2010, you’re likely still on rsa-sha1. It’s not negligence—it’s just how systems evolve. But that doesn’t mean you can’t fix it.

If you’re unsure whether your domain’s DKIM configuration is up to date, test your setup with actual inbox placement tools. MailTester’s inbox placement checker simulates delivery across real providers and flags known cryptographic issues before you send to real users.

How to Test That Your DKIM Fix Works Across Providers

Send test emails to Gmail, Outlook, Yahoo, and Apple inboxes, then use MailTester’s inbox placement tool to check the email headers. A properly fixed DKIM signature will now show a=rsa-sha256 in the verification status, confirming the older a=rsa-sha1 algorithm is no longer in use. This cross-provider validation ensures your authentication works consistently, not just in one provider’s environment.

Step-by-Step: Verify DKIM Fixes Across Major Inboxes

Send a test email from your server to at least four major inbox providers: gmail.com, outlook.com, yahoo.com, and apple.com. This tests real-world delivery paths, including different filtering logic, alignment rules, and DMARC enforcement practices used by each service.
Use MailTester’s inbox placement tool to analyze the full header of the received message. This tool checks sender reputation, SPF/DKIM/DMARC alignment, and spam score. It shows exactly how each provider treated your email, including whether DKIM verification passed and which algorithm was used.
Inspect the DKIM signature in the email headers—look for the d= and a= values. If the fix worked, a=rsa-sha256 must appear, not a=rsa-sha1. An outdated algorithm will fail verification on modern systems, even if the key is valid.
Review all authentication records in the headers: ensure SPF passes, DKIM aligns with the From domain, and DMARC policy is enforced. Misalignment or policy failure can cause messages to be marked as spam or rejected despite a valid DKIM signature.
Repeat across multiple domains to ensure consistent results. Some providers still allow legacy algorithms temporarily, but they’re being phased out. For example, RFC 8301 (published by IETF) mandates the use of stronger hash algorithms like SHA-256 for cryptographic signatures.

Why This Matters: Real-World Delivery Isn't Just About Correct Headers

Even if your DKIM signature passes in isolation, many email providers apply additional checks. Google, for example, uses a combination of reputation score, engagement history, and header validation to determine inbox placement. A failed DKIM verification on one provider may not break delivery on another, but consistent failures across multiple providers indicate a systemic problem.

The 5 steps described in “Step-by-Step: Verify DKIM Fixes Across Major Inboxes”, in order.

Use MailTester’s inbox placement tester to simulate real delivery conditions and validate that your fixes apply uniformly. This includes checking for role accounts, temporary domains, or greylisting behavior that might mask underlying issues.

Fixing DKIM Isn’t Just Technical — It’s a Deliverability Imperative

Even a single outdated DKIM setting like a=rsa-sha1 can cause messages to be rejected or marked as spam. What starts as a technical detail quickly becomes a campaign failure, lower open rates, and lost revenue.

Fixes like updating the digital signature algorithm aren’t complex — but delay invites disaster. Waiting for bounces or delivery issues means your audience has already been lost. Preventing the problem is faster and more reliable than cleaning up after it.

Use real-time verification tools to audit your sending setup.
Test inbox placement before your next campaign launches.
Verify every list with tools that check SPF, DKIM, DMARC, and domain health.

Sources

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

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Frequently asked questions

Does every email with a=rsa-sha1 get blocked?

No, but many modern providers reject or flag messages using this algorithm. Even if delivered, it harms sender reputation and may lead to filtering over time.

Can I fix a=rsa-sha1 without changing my email server?

Yes, if your email provider supports it. Most modern platforms (SendGrid, Mailchimp) allow you to switch the DKIM algorithm via their dashboard settings.

How long does it take to see deliverability improvements after fixing DKIM?

Improvements can appear within 24–72 hours, but full reputation recovery may take several weeks depending on your sending volume and consistency.

Is a=rsa-sha256 the only modern DKIM algorithm?

It’s the most widely adopted. Alternatives like a=rsa-sha256 and a=ed25519 exist, but a=rsa-sha256 is required by all major providers today.

Can I still use email tools that default to a=rsa-sha1?

You can, but only if you confirm they support algorithm updates. Otherwise, your emails may fail authentication or be marked as risky.

What happens to old emails sent with a=rsa-sha1?

They remain in recipient mailboxes but may be flagged during message analysis by providers. They don’t harm future emails directly, but they contribute to a negative sender profile.

How do I know if my DKIM record is valid?

Use a DNS lookup tool or MailTester’s real-time verification API to test the record and verify the signature passes validation.

Does DMARC help with a=rsa-sha1 issues?

DMARC doesn’t fix DKIM algorithm issues — it only controls how failures are reported. You still need a modern DKIM signature to pass authentication.

Are there tools that detect deprecated DKIM algorithms automatically?

Yes — tools like MailTester scan authentication headers and detect deprecated algorithms like a=rsa-sha1 before you send.

Can I test DKIM changes without sending real emails?

Yes — use MailTester’s inbox placement testing feature to emulate real delivery without triggering actual sends to your list.

Do I need to update both SPF and DKIM?

Only DKIM needs updating for the signature algorithm. SPF remains independent and unchanged unless you alter your sending setup.

Why is a=rsa-sha1 considered insecure?

SHA-1 has known cryptographic weaknesses that allow attackers to generate fake signatures. It is no longer considered trustworthy for email authentication.