What causes DKIM key issues when DNS provider limits TXT record length?
Michael Ko
Co-founder & CEO, Suped
Published 20 Apr 2025
Updated 19 Aug 2025
8 min read
Many email senders encounter issues when trying to publish their DKIM public keys in their domain's DNS. A common culprit is a DNS provider that limits the length of TXT records. While the DNS protocol itself allows for TXT records longer than 255 characters by segmenting the data, some older or less sophisticated DNS management interfaces fail to handle this correctly. This can lead to frustration and, more importantly, email authentication failures.
Understanding why this happens is crucial for maintaining good email deliverability. I'll explain the underlying causes of these DKIM key issues and provide practical solutions to ensure your emails are properly authenticated and reach their intended recipients.
DKIM (DomainKeys Identified Mail) is an email authentication method that uses cryptographic signatures to verify the sender's identity and ensure that the email content has not been tampered with during transit. When an email is sent, the sending mail server signs it with a private key. The corresponding public key is published in your domain's DNS as a TXT record. Receiving mail servers then retrieve this public key to verify the email's signature.
TXT records are a type of DNS record that contains human-readable text information for external sources. They are commonly used for various purposes, including SPF (Sender Policy Framework), DMARC (Domain-based Message Authentication, Reporting, and Conformance), and, of course, DKIM. The issue arises because while the DNS specification allows a single TXT record to contain multiple character strings, each individual string segment is typically limited to 255 characters.
When a DKIM public key is generated, especially for 2048-bit keys, it often exceeds this 255-character limit for a single string. DNS resolvers are designed to concatenate these multiple strings (enclosed in double quotes) within a single TXT record into one continuous string. However, many DNS providers' web interfaces or older systems do not properly support entering multiple quoted strings in a single TXT record field, or they impose an overall character limit on the entire record.
Example DKIM TXT record structure (hypothetical)DNS
v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQD7hG6/4e3o8u+f... (truncated for brevity) ...l2k1e4qN6/K8L9mK6f7t8u9v0w1x2y3z4A5B6C7D8E9F0G1H2I3J4K5L6M7N8O9P0Q1R2S3T4U5V6W7X8Y9Z+A/B/C/D/E/F/G/H/I/J/K/L/M/N/O/P/Q/R/S/T/U/V/W/X/Y/Z;
Decoding the TXT record length limit
The root of the problem often lies in how DNS providers implement their record management interfaces. Many interfaces are designed for simpler, shorter TXT records and don't account for the longer, multi-string format required by modern DKIM keys (especially 2048-bit ones, which are recommended for better security). If a provider's interface or backend system automatically truncates the key or misinterprets the multiple strings as separate records, your DKIM signature will fail validation.
Google Workspace and other major email providers recommend using 2048-bit DKIM keys for enhanced security. However, these longer keys inherently produce a public key string that extends beyond the common 255-character per-string limit. If your DNS provider's system doesn't correctly concatenate multiple strings within a single TXT record, the published key will be incomplete or malformed.
This misconfiguration can lead to severe email deliverability issues. When a receiving server attempts to verify your email's DKIM signature, it will fetch the incomplete or malformed public key, resulting in a DKIM authentication failure. This failure can cause your emails to be marked as spam, sent to junk folders, or even rejected outright by the recipient's mail server, impacting your sender reputation and campaign effectiveness.
The impact of a truncated DKIM key
When your DKIM key is cut short or incorrectly entered due to a DNS provider's TXT record length limitations, it directly impacts your email's authenticity. This can lead to a range of negative consequences for your email program.
Authentication failures: Receiving mail servers cannot validate your email's signature, leading to a DKIM fail.
Reduced deliverability: Emails are more likely to land in spam folders or be rejected by ISPs.
Damaged sender reputation: Consistent DKIM failures signal untrustworthiness to receiving mail servers.
DMARC policy enforcement: If you have a DMARC policy (even a lenient one), DKIM failures can lead to emails being quarantined or rejected according to your policy settings. Learn more about DMARC verification failures.
Strategies for long DKIM keys
The most common and correct solution for long DKIM keys is to split the public key string into multiple segments. Each segment should be enclosed in its own set of double quotes, and all segments should be placed within the same single TXT record. DNS resolvers will then automatically concatenate these segments to form the complete public key.
For example, if your DKIM key is 300 characters long, you would split it into two strings: one of 255 characters and the remaining 45 characters. Both strings would reside in the same TXT record, appearing as "part1""part2". It's important to differentiate this from creating multiple, separate TXT records for the same DKIM selector, which will not work and will lead to validation errors.
Example of a split DKIM TXT recordDNS
yourselector._domainkey.yourdomain.com. IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQD7hG6/4e3o8u+f0g1h2j3k4l5m6n7o8p9q0r1s2t3u4v5w6x7y8z9A0B1C2D3E4F5G6H7I8J9K0L1M2N3O4P5Q6R7S8T9U0V1W2X3Y4Z5A6B7C8D9E0F1G2H3I4J5K6L7M8N8O9P0Q1R2S3T4U5V6W7X8Y9Z+A/B/C/D/E/F/G/H/I/J/K/L/M/N/O/P/Q/R/S/T/U/V/W/X/Y/Z+" "aAbBcCdDeEfFgGhHiIjJkKlLmMnNoOpPqQrRsStTuUvVwWxXyYzZ0123456789/="
If your DNS provider's interface actively prevents you from entering the key in this multi-string format, you might need to consider migrating your DNS hosting to a provider that offers more robust and compliant DNS management. Many modern DNS services, including those from major cloud providers, handle long TXT records effortlessly. This can often resolve issues not just for DKIM, but also for other records like SPF that might approach or exceed character limits.
For specific guidance on managing other DNS record length issues, you can refer to our guides on why your SPF record might be too long or what to do if your provider has a smaller limit. Some providers may also offer CNAME delegation, which can simplify DKIM management by pointing to a key hosted by your email service provider, circumventing local TXT record limits.
Avoiding DKIM key issues and ensuring deliverability
When your DKIM key is not correctly published, it can significantly impact email deliverability. Mailbox providers heavily rely on DKIM authentication to filter out spam and phishing attempts. A failing DKIM signature often results in your legitimate emails being misclassified, ending up in spam folders, or being rejected entirely. This directly contributes to poor inbox placement and a decline in your sender reputation.
Regularly monitoring your DMARC reports is essential to catch DKIM failures early. These reports provide detailed insights into your email authentication status, helping you identify if DKIM is failing and from which sources. Pay close attention to error messages or bounce codes, such as the 4.4.4 (unable to route: dns lookup failure) message, which, while not always directly DKIM-related, can indicate underlying DNS problems that might affect your DKIM record's visibility.
If you suspect a DKIM key issue due to DNS provider limits, here's a quick checklist to follow:
Review your DKIM key: Confirm its length and compare it to your DNS provider's TXT record limits. Most 2048-bit keys will exceed 255 characters.
Check your DNS provider's documentation: Look for specific instructions on how to handle long TXT records or DKIM keys.
Implement multi-string TXT records: Split your key into 255-character segments and enclose each in quotes within the same record.
Consider DNS migration: If your current provider remains a barrier, switch to a more capable DNS host. This is a sound long-term investment for email stability.
Always use 2048-bit DKIM keys for enhanced security, even if they are longer.
Split long DKIM keys into multiple 255-character strings within the same TXT record, enclosed in quotes.
Regularly check your published DKIM record for correct formatting and propagation.
Consider using a modern, reliable DNS provider if yours has problematic interface limitations.
Common pitfalls
Creating multiple separate TXT records for the same DKIM selector instead of splitting one.
Assuming your DNS provider's interface automatically handles long TXT records.
Ignoring DKIM authentication failures reported in DMARC reports, leading to deliverability issues.
Using outdated or weak 1024-bit DKIM keys just to fit length limits, compromising security.
Expert tips
If your DNS provider's portal limits input, sometimes the character restriction is client-side only and can be bypassed by advanced UI interactions, though this requires technical expertise.
For organizations with complex setups or hybrid environments, an internal DNS resolver might cause lookup failures if it's misconfigured or not pointing to external DNS correctly.
Even if your DNS provider claims a lower limit, the DNS protocol itself allows concatenated strings within a single TXT record, so push for proper implementation if you face resistance.
Using a CDN or security service that also offers DNS management (e.g., Cloudflare) can be a simple way to manage your DNS records elsewhere if your current provider is inflexible.
Expert view
Expert from Email Geeks says many companies providing DNS services aren't truly competent DNS providers and often offer inaccurate advice.
2021-05-25 - Email Geeks
Expert view
Expert from Email Geeks says duplicate TXT entries for a DKIM key will result in an invalid DKIM key and, consequently, no valid DKIM signatures.
2021-05-25 - Email Geeks
Ensuring proper DKIM authentication
Dealing with DKIM key issues due to DNS provider limits on TXT record length can be a frustrating experience, but it's a solvable problem. By understanding that the limitation often lies with the DNS management interface rather than the DNS protocol itself, you can implement effective workarounds.
Properly splitting your DKIM public key into multiple quoted strings within a single TXT record is the standard approach. If your current DNS provider doesn't support this, don't hesitate to explore alternative DNS hosting solutions that offer more flexibility and adhere to modern DNS standards. Ensuring your DKIM records are correctly published is a fundamental step in achieving excellent email deliverability and protecting your sender reputation from being placed on a blocklist (or blacklist).
