Summary
Email providers are becoming increasingly strict regarding the strength and type of DKIM keys used for email authentication. They handle 'weak' DKIM keys, typically those under 1024 bits, with suspicion, often leading to increased spam scoring, deliverability issues, or outright rejection of messages. Conversely, 'exotic' DKIM keys, referring to non-RSA algorithms or non-standard exponents, are generally not supported or recognized by current email standards and receiving mail servers, almost certainly resulting in authentication failures. Best practices and provider requirements consistently point towards using robust 1024-bit or, preferably, 2048-bit RSA keys to ensure optimal email deliverability and maintain a strong sender reputation.
Key findings
- Weak key rejection: Many major email providers are increasingly rejecting or significantly penalizing emails signed with cryptographically weak DKIM keys (e.g., those under 1024 bits, particularly 512-bit keys). Some providers, including Yahoo, have explicitly stated requirements for 2048-bit keys, effectively making 1024-bit keys less secure in their eyes.
- Increased spam scoring: Emails with weak DKIM signatures, even if technically valid, are often assigned higher spam scores by receiving mail servers. This increases the likelihood of messages being delivered to the spam or junk folder, as providers consider such keys to be a security risk or a sign of outdated practices.
- Lack of exotic key support: Email providers primarily support DKIM signatures using the RSA algorithm. 'Exotic' or non-standard key types and non-default exponents are generally not recognized or correctly processed by compliant mail servers, resulting in DKIM authentication failures and a negative impact on deliverability.
- Vulnerability implications: Shorter DKIM keys are known to be more susceptible to brute-force attacks and cryptographic exploits. This inherent weakness leads email providers to treat signatures using these keys as suspicious, invalid, or simply disregard them due to the ease with which they could be forged.
Key considerations
- Key strength: Prioritise using strong DKIM keys, specifically 1024-bit or, preferably, 2048-bit RSA keys. This adheres to industry best practices and is increasingly required by major email providers like Yahoo and Verizon Media to ensure proper authentication.
- Standard algorithm support: Avoid using DKIM signatures with 'exotic' or non-standard algorithms, such as ed25519, or non-default exponents. The only officially defined and widely supported DKIM key type is RSA (k=rsa); using anything else will likely result in validation failures and impact your email deliverability.
- Sender reputation: Understand that utilizing weak or non-standard DKIM keys can significantly harm your sender reputation. Email providers view such practices as indicators of poor security, which can lead to higher spam scores and messages landing in the spam folder, even if other authentication methods pass.
What email marketers say
12 marketer opinions
Email providers, using increasingly sophisticated scoring systems, treat DKIM keys under 1024 bits (often called 'weak' keys) with suspicion, even if they are technically valid. This frequently results in increased spam scoring, deliverability challenges, or outright message rejection. Similarly, 'exotic' DKIM keys, such as those employing non-RSA algorithms (e.g., ed25519) or non-standard exponents, are largely unsupported or unrecognized by receiving mail servers, leading to authentication failures. These practices collectively signal poor security or misconfiguration, negatively affecting a sender's reputation and their ability to reach the inbox.
Key opinions
- Weak keys are penalized: While some older systems may still generate 512-bit DKIM keys, modern email providers (like Gmail) consider them cryptographically weak, even if technically valid. This leads to increased spam scoring or rejection, effectively penalizing senders for not using at least 1024-bit keys.
- Practical weakness: DKIM keys shorter than 1024 bits significantly increase the chance of emails being marked as spam or facing validation failures due to their cryptographic vulnerability. Major providers apply additional scrutiny beyond mere technical adherence to standards.
- Unsupported exotic algorithms: Algorithms beyond RSA, such as ed25519, and deviations from the standard DKIM exponent (65537), are generally not supported by current email infrastructure. Using these 'exotic' methods almost certainly results in DKIM authentication failures.
- Reputational impact: Employing weak DKIM keys is perceived as a sign of poor security practices by email providers. This negative perception can harm a sender's overall reputation, causing emails to be blocklisted or land in spam folders, even if other authentication checks pass.
Key considerations
- Prioritise robust key strength: Always use DKIM keys of at least 1024 bits, with 2048 bits being the recommended standard for enhanced security and optimal deliverability. This aligns with modern cryptographic requirements and provider expectations.
- Adhere to standard algorithms: Limit DKIM key usage to the widely accepted RSA algorithm and the standard exponent of 65537. Non-standard or less common algorithms are unlikely to be properly validated by most receiving mail servers.
- Understand provider scrutiny: Be aware that email providers implement complex scoring systems that go beyond basic authentication checks. Weak or exotic DKIM implementations are flags that can trigger higher spam scores or outright rejections, irrespective of other passing authentication methods.
- Continuous monitoring: Regularly review your email authentication results to ensure DKIM is consistently passing for all your outgoing mail. This helps quickly identify and rectify any issues related to key strength or type that might impact deliverability.
Marketer view
Marketer from Email Geeks explains they are conducting a project with their CEO to study how different email providers handle exotic and weak DKIM keys and is requesting assistance from others to send test emails to non-listed providers and return Authentication-Results headers.
27 Feb 2025 - Email Geeks
Marketer view
Marketer from Email Geeks responds by offering to assist with the DKIM key testing project via direct message.
6 Jul 2025 - Email Geeks
What the experts say
2 expert opinions
Email providers are now strictly enforcing higher standards for DKIM key strength, with prominent services like Yahoo and Verizon Media explicitly requiring 2048-bit DKIM keys for all outgoing mail. This policy shift means that emails signed with weaker 1024-bit keys may be rejected outright or encounter deliverability obstacles. These measures underscore a commitment to robust authentication, minimizing the chance of email abuse and fostering a more secure email environment.
Key opinions
- Explicit 2048-bit requirement: Major email providers, including Yahoo and Verizon Media, have publicly stated their requirement for senders to use 2048-bit DKIM keys for all outbound email. This is a clear shift towards stronger authentication.
- Rejection of 1024-bit keys: Mail signed with 1024-bit DKIM keys, previously a common standard, is now often rejected or experiences significant deliverability challenges, as providers view them as increasingly less secure.
- Promotion of stronger authentication: By rejecting weaker DKIM implementations, email providers are actively promoting and enforcing the adoption of more robust authentication standards across the email ecosystem, impacting both blocklist status and inbox placement.
Key considerations
- Adopt 2048-bit keys as standard: Ensure all outgoing email is authenticated with 2048-bit DKIM keys. This is rapidly becoming a mandatory requirement for major email providers, critical for maintaining strong deliverability and avoiding blocklist issues.
- Retire weaker keys: Proactively identify and update any DKIM keys shorter than 2048 bits, particularly 1024-bit keys. While once acceptable, these are now frequently rejected or result in messages being flagged by receiving mail servers.
- Align with evolving policies: Keep abreast of explicit authentication requirements from major email providers (such as Yahoo and Verizon Media) to ensure your email sending practices remain compliant and your messages consistently reach the inbox.
Expert view
Expert from Word to the Wise shares that email providers are becoming increasingly strict about authentication standards. For instance, Yahoo and Verizon Media have publicly stated that they require senders to use 2048-bit DKIM keys for all outbound mail. This indicates that weaker 1024-bit keys may be rejected or result in deliverability issues, effectively addressing how providers handle less secure DKIM implementations.
9 Jan 2023 - Word to the Wise
Expert view
Expert from Spam Resource explains that major email providers, such as Yahoo, have long enforced stricter requirements for DKIM key sizes. Specifically, mail signed with a weaker 1024-bit DKIM key was rejected, while a 2048-bit key was required. This policy demonstrates that providers handle weak or less secure DKIM keys by simply not accepting mail signed with them, thereby promoting stronger authentication standards.
15 Aug 2022 - Spam Resource
What the documentation says
7 technical articles
Building on the evolving landscape of email security, providers are rigorously assessing incoming messages based on both the cryptographic strength and specific type of DKIM keys used. Keys deemed 'weak,' typically falling below the recommended 1024-bit length, are met with increased skepticism, often leading to reduced inbox placement or even outright rejection. Similarly, 'exotic' DKIM key types—those employing non-RSA algorithms or non-standard exponents—are largely incompatible with the established DKIM standard, resulting in authentication failures. This stringent evaluation reflects a broader effort to enhance email security and combat potential abuse.
Key findings
- Key length scrutiny: Email providers, including Microsoft 365, explicitly support DKIM with 1024-bit and 2048-bit RSA keys. This implies that emails signed with shorter or unsupported key lengths may not be correctly processed, or could even be disregarded, affecting deliverability.
- Vulnerability of weak keys: Research indicates that DKIM keys using shorter lengths, such as 512 bits, are significantly more vulnerable to cryptographic attacks. This inherent weakness causes email providers to treat such signatures as highly suspicious or invalid, impacting how your messages are handled.
- Non-standard algorithm rejection: The IETF RFC 6376 and the IANA registry specify that RSA (k=rsa) is the only officially recognized and defined key type for DKIM. Consequently, any 'exotic' or non-RSA algorithm used in a DKIM signature will be considered non-compliant, leading to validation failures by receiving mail servers.
- Importance of best practices: Leading industry bodies like M3AAWG, alongside platforms such as Mailchimp, consistently advise using a minimum of 1024-bit DKIM keys, with 2048 bits recommended. Adhering to these best practices helps ensure your emails are properly authenticated and achieve optimal inbox placement.
Key considerations
- Adopt strong DKIM keys: To ensure optimal email deliverability and strong authentication, always use DKIM keys that are at least 1024 bits in length, with 2048 bits being the preferred standard. This aligns with current security recommendations and provider expectations.
- Adhere to standard key types: Ensure your DKIM setup uses the RSA algorithm, as it is the universally accepted standard. Employing 'exotic' or non-RSA key types will likely result in authentication failures, leading to a negative impact on your email's reputation and potential blocklist issues.
- Protect sender reputation: Be aware that using weak or non-compliant DKIM keys can severely harm your sender reputation. Email providers view such practices as indicators of poor security, which can cause your messages to be flagged, increasing their likelihood of landing in spam folders or on a blacklist.
Technical article
Documentation from M3AAWG's Sender Best Current Practices states that DKIM keys should be a minimum of 1024 bits for adequate security, with 2048 bits recommended for higher security. This implies that email providers, adhering to industry best practices, will consider shorter keys weak and may treat emails signed with them less favorably, impacting deliverability.
6 Dec 2024 - M3AAWG Sender Best Current Practices
Technical article
Documentation from Microsoft Learn states that Microsoft 365 supports DKIM with 1024-bit and 2048-bit RSA keys. This indicates that email providers like Microsoft may not correctly process or may disregard DKIM signatures using shorter or unsupported key lengths.
22 Dec 2022 - Microsoft Learn
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