Is SHA1 DKIM key still secure and does it affect email deliverability?
Matthew Whittaker
Co-founder & CTO, Suped
Published 12 Aug 2025
Updated 19 Aug 2025
8 min read
Email authentication is a cornerstone of modern email deliverability, and DKIM (DomainKeys Identified Mail) plays a vital role in this. DKIM allows a sender to cryptographically sign emails, which receiving mail servers can then verify. This signature helps to assure recipients that the email has not been tampered with in transit and that it genuinely originates from the stated domain. Without proper authentication, your emails face a high risk of landing in spam folders or being rejected outright, negatively impacting your email deliverability. One question that often arises, especially when dealing with older email infrastructure, is the security and deliverability impact of using SHA1 DKIM keys.
The SHA1 hashing algorithm was once a standard, but cryptographic advancements have exposed its vulnerabilities. This leads to concerns: Is a DKIM key using SHA1 still considered secure? More importantly, does its use affect whether your legitimate emails reach the inbox? Let's explore the current standing of SHA1 DKIM keys and their implications for your email program.
At its core, DKIM relies on cryptographic hashing algorithms to create a unique digital signature for each email. When an email is sent, the sending server generates a hash of certain parts of the email (like headers and body) using a specified algorithm. This hash is then encrypted with the sender's private key and included in the email's DKIM-Signature header. The recipient server uses the corresponding public key, found in the domain's DNS records, to decrypt the hash and verify the email's integrity. DKIM's main purpose is to verify that the content was not altered in transit.
Historically, SHA1 (Secure Hash Algorithm 1) was a common choice for this hashing. However, cryptographic research has demonstrated that SHA1 is vulnerable to collision attacks, meaning it's computationally feasible for an attacker to create two different inputs that produce the same SHA1 hash. This vulnerability undermines the integrity assurance that DKIM is designed to provide. If an attacker can create a malicious email with the same SHA1 hash as a legitimate one, they could potentially forge a valid DKIM signature, making their malicious email appear legitimate.
Due to these security concerns, the internet community has been moving away from SHA1 for various cryptographic applications, including DKIM. While an SHA1 DKIM key might still technically pass validation checks by many receiving servers today (because it is still a valid, albeit deprecated, algorithm), it does not mean it is trusted or provides sufficient security. RFC 8301, which outlines recommendations for DKIM signing, deprecates SHA1 and states that DKIM signatures identified as signed with historic algorithms (like rsa-sha1) should permanently fail evaluation.
Security and deliverability implications
While an SHA1 DKIM signature might still pass initial validation, its inherent insecurity poses a significant threat to your email program's integrity and a potential deliverability red flag. Mailbox providers, such as Google and Yahoo, are increasingly tightening their email authentication requirements to combat spam and phishing. They look for strong authentication signals to trust incoming mail. An outdated or vulnerable cryptographic algorithm is a weak signal at best, and a red flag at worst.
Even if your email is not immediately rejected, using a deprecated SHA1 key can lead to degraded inbox placement. Mailbox providers might assign a lower trust score to emails signed with SHA1, pushing them into the spam folder, or even an untrusted bulk folder, rather than the primary inbox. This is especially true as more and more major email providers enforce stricter authentication policies, considering any security weakness a reason to deprioritize or filter traffic.
Beyond direct deliverability impact, using SHA1 can harm your domain's sender reputation. A reputation for using outdated security protocols can make your domain appear less legitimate to receiving mail servers. This can lead to your emails being added to internal blacklists (or blocklists) managed by ISPs, even if your domain isn't on a public blocklist (or blacklist). Proactive adoption of modern security standards signals to ISPs that you are a responsible sender, which is crucial for maintaining excellent inbox placement.
Recommendations for modern DKIM implementation
Given the deprecation of SHA1 and the evolving landscape of email security, upgrading your DKIM key to SHA256 (preferably with a 2048-bit key length) is not just a recommendation, but a necessity for robust email deliverability and security. While an SHA1 key might still pass validation today, the risk of it being treated as untrusted, or leading to a perceived lack of authentication, is increasing.
The problem with SHA1 DKIM
Vulnerability to attacks: SHA1 is susceptible to collision attacks, meaning attackers could forge signatures.
Future deprecation: Formal deprecation means it will eventually lead to hard failures for email validation.
Upgrading to SHA256 is generally straightforward and involves generating new keys and updating your DKIM DNS record. Most modern mail servers and email service providers support SHA256, and the transition typically causes no disruption if managed correctly. Remember that DKIM key rotation is a best practice, so this upgrade can be part of your regular security maintenance schedule. For new setups, always use SHA256.
Furthermore, consider the key length. While 1024-bit keys were once standard, 2048-bit keys offer superior security and are increasingly recommended, especially by major providers like Google. Adopting a 2048-bit SHA256 key ensures your DKIM signatures are robust and future-proof against evolving threats and stricter ISP requirements. It's a proactive step that will benefit your email reputation and deliverability in the long run.
The path to better security and deliverability
SHA1 DKIM keys
Security status: Cryptographically vulnerable to collision attacks.
RFC status: Deprecated in RFC 8301, considered historic.
Deliverability impact: May still pass, but risks lower trust, increased spam filtering, or outright rejection by strict receivers. It's a potential flag for blacklisting.
Recommendation: Upgrade immediately to SHA256.
SHA256 DKIM keys
Security status: Considered cryptographically strong and secure.
RFC status: Recommended algorithm for new DKIM implementations.
Deliverability impact: Improves trust, increases inbox placement rates, and aligns with modern ISP requirements.
Recommendation: Standard for all email authentication setups.
If you are running an older SMTP server and have identified a SHA1 DKIM key, immediate action is advised. While it might still function, it exposes your email streams to unnecessary security risks and could be silently impacting your email deliverability by lowering your sender trust score with major ISPs. Upgrading to SHA256 with a 2048-bit key is a critical step in modernizing your email authentication and ensuring your messages reliably reach their intended recipients.
The process of updating your DKIM key typically involves generating a new key pair using the SHA256 algorithm, updating your DNS TXT record with the new public key, and configuring your mail server or email service provider to sign outgoing emails with the new private key. This is a standard procedure and most platforms offer clear guidance on how to do this. After making the change, closely monitor your DMARC reports to confirm successful implementation and observe any changes in deliverability metrics.
Views from the trenches
Best practices
Always use SHA256 for new DKIM implementations to ensure strong cryptographic security and future compatibility.
Choose a 2048-bit key length for optimal security, as recommended by major email providers like Google.
Implement a regular DKIM key rotation schedule (e.g., every 3-6 months) to mitigate risks of key compromise.
Monitor your DMARC reports closely after any DKIM key changes to verify successful implementation and prevent deliverability issues.
Common pitfalls
Delaying the upgrade from SHA1, risking lower trust scores and increased spam filtering by receiving servers.
Assuming that a 'passing' SHA1 DKIM signature means it's fully trusted by all ISPs, leading to hidden deliverability problems.
Neglecting to rotate DKIM keys, which can lead to security vulnerabilities over time.
Using insufficient key lengths (e.g., 1024-bit) when 2048-bit keys are preferred for modern security standards.
Expert tips
If your SMTP server is old, it might be challenging to update your DKIM keys to modern standards.
Upgrading your DKIM key might not yield immediate, dramatic deliverability changes, but it's crucial for long-term stability.
It's easier to prevent deliverability problems by having updated security than trying to fix them once your domain reputation is damaged.
Consider a vulnerable DKIM algorithm almost as detrimental as having no DKIM at all, especially with stringent filters.
Expert view
Expert from Email Geeks says they would not recommend using an old, insecure key type, as it can indeed affect deliverability.
2025-06-24 - Email Geeks
Marketer view
Marketer from Email Geeks says that if an algorithm isn't trusted to be secure, it's an issue, especially with platforms like Gmail having no tolerance for security deficiencies.
2025-06-24 - Email Geeks
Moving forward with stronger DKIM
While an SHA1 DKIM key may technically still pass validation on some systems, it is cryptographically weak and has been officially deprecated. Its continued use introduces security vulnerabilities that can be exploited, potentially leading to spoofing and phishing attacks. More importantly for email marketers, it can severely impact email deliverability by eroding sender trust with major mailbox providers.
The clear recommendation is to upgrade your DKIM keys to SHA256, preferably with a 2048-bit length, as soon as possible. This proactive measure not only enhances the security of your email communications but also aligns your domain with current industry best practices, safeguarding your sender reputation and optimizing your inbox placement. In the ever-evolving landscape of email security, staying ahead of deprecated standards is crucial for consistent and reliable email delivery.
