Is it problematic to use the same DKIM domain and selector for multiple email campaigns, and why do validation tools sometimes show errors for valid DKIM records?
Michael Ko
Co-founder & CEO, Suped
Published 22 Apr 2025
Updated 19 Aug 2025
7 min read
Email authentication protocols like DKIM (DomainKeys Identified Mail) are foundational to ensuring your messages reach the inbox. It helps recipient mail servers verify that an email was sent by the domain owner and hasn't been tampered with in transit. This is crucial for building and maintaining a good sender reputation, which directly impacts your email deliverability.
A common scenario for many organizations is using a single domain for various email purposes, from marketing newsletters to transactional emails and customer support. This often leads to questions about how DKIM should be set up, especially regarding the use of the same DKIM domain and selector across these different campaigns.
In this article, I'll explore whether it's problematic to use the same DKIM domain and selector for multiple email campaigns and delve into the reasons why validation tools sometimes report errors for what appear to be valid DKIM records. Understanding these nuances is key to optimizing your email sending infrastructure and ensuring consistent inbox placement.
DKIM works by attaching a digital signature to outgoing emails, which is then verified by the recipient's mail server against a public key published in your domain's DNS records. The signature includes a d= tag, which indicates the signing domain, and an s= tag, which is the selector. This selector is a unique name that identifies the specific public key used to verify the signature. You can learn more about these tags in our guide on DMARC, SPF, and DKIM.
The d= tag is fundamentally linked to your sender reputation. When a receiving server evaluates an email, it looks at the signing domain specified in the DKIM signature to assess the email's trustworthiness. This means that consistent authentication for your primary sending domain, regardless of the specific campaign, is critical for positive perception.
The selector, on the other hand, provides flexibility. It allows you to have multiple DKIM keys for a single domain. For example, you might use different selectors if you have multiple Email Service Providers (ESPs) sending on behalf of your domain, or if you rotate your keys for security purposes. This concept is explored further in our guide on common DKIM selectors and their use.
Sharing DKIM domains and selectors across campaigns
It is generally not problematic to use the same DKIM domain and selector for multiple email campaigns, provided that your email sending system is configured correctly to apply the signature for all messages. The crucial element is the d= tag, as this is what most mail receivers use to key reputation.
If you use the same DKIM domain and selector, all your email campaigns (marketing, transactional, etc.) will share the same sender reputation. This can be an advantage if all your campaigns have high engagement and low complaint rates, as they will mutually benefit from a strong reputation. However, it also means that poor sending practices in one campaign can negatively affect the deliverability of all others. To understand how shared DKIM keys affect deliverability, you can refer to our article on individual versus shared DKIM.
Some organizations opt for different selectors for different sending platforms or campaign types to isolate their reputation. For instance, if you use SendGrid for transactional emails and another ESP for marketing, you might use a unique selector for each. This allows you to monitor and manage the reputation of each email stream independently. Twilio SendGrid documentation provides a good overview of setting up domain authentication, including custom selectors. Also, our article on multiple DKIM records for multiple ESPs can offer further insights.
Shared DKIM domain and selector
Reputation Pooling: All campaigns contribute to and draw from a single sender reputation. Good practices from one campaign can lift others, and poor practices can harm all.
Simplicity: Easier DNS management with fewer records to maintain.
Interdependency: A deliverability issue (e.g., a spam blocklist entry) affecting one campaign can directly impact all others using the same DKIM setup.
Separate DKIM selectors
Reputation Segmentation: Allows distinct reputations for different sending streams, isolating potential issues.
Complexity: Requires more DNS records and careful management of each selector's key.
Granular Control: Provides better insight into the performance of individual campaign types and allows for targeted troubleshooting.
Why DKIM validation tools show errors
Even when your DKIM record is correctly configured and your emails are passing authentication checks at receiving mail servers, some online validation tools may still report errors. This discrepancy can be confusing and lead to unnecessary troubleshooting.
One common reason for these false positives is strict parsing by the validation tool itself. For instance, extra spaces within the public key string (the p= tag value) or other minor formatting inconsistencies can cause a checker to flag an error, even if a receiving Mail Transfer Agent (MTA) would silently ignore it. The RFC 6376 specification for DKIM outlines the parsing rules, but interpretations can vary.
Common reasons for validation tool errors
Sometimes, validation tools can be overly sensitive to minor formatting issues in your DKIM TXT record. These include:
Extra Spaces: Spaces within the public key string (e.g., between characters in the p= tag) or at the end of the TXT record can cause errors with some strict parsers. Most real-world MTAs are more forgiving because they process the base64 encoded data, which often discards such extraneous spaces.
Missing Version Tag: While the DKIM version tag (v=DKIM1) is recommended, its absence often doesn't break DKIM verification by receiving servers, yet some tools might flag it as missing or invalid.
DNS Propagation Delays: After updating your DNS records, it takes time for changes to propagate across the internet. A validation tool might check the record before the new version has fully propagated, leading to a temporary error.
Casing Inconsistency: Though less common, some tools might be sensitive to the casing of the selector or other tags, even though DNS lookups are generally case-insensitive. You can learn more in our article Is DKIM signature case-sensitive?
Here's an example of a potentially problematic DKIM record with an accidental space, which some tools might flag:
DKIM TXT record with an accidental space
mktg._domainkey.yourdomain.com IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDzQ8Lz 4Y5K9e..."
The space after z4Y5K9e in the p= value is the issue. While some lenient parsers might ignore it, stricter ones will consider the record invalid. This is why it's important to cross-reference multiple tools and check the actual email headers.
Best practices for DKIM configuration and troubleshooting
To ensure your DKIM setup is robust and avoids false positives from validation tools, always adhere to best practices. This includes careful attention to the DNS record format and consistent monitoring.
When creating or updating your DKIM DNS record, ensure there are no unintended spaces or characters, particularly within the public key string. If you're using a tool to generate your DKIM record, copy and paste the output precisely. Regularly checking your DKIM setup using multiple reputable online DKIM checkers can help identify inconsistencies.
Implementing DMARC (Domain-based Message Authentication, Reporting, and Conformance) is also crucial. DMARC relies on SPF and DKIM to authenticate your emails and provides powerful reporting capabilities. These reports can show you exactly how different receiving mail servers are validating your DKIM signatures, helping you identify real issues versus tool-specific errors. For more information, see our guide on understanding and troubleshooting DMARC reports.
DKIM state
Implication for deliverability
Troubleshooting steps
DKIM pass
Good sender reputation, improved inbox placement. Enables DMARC alignment.
Emails may go to spam or be rejected. Impacts sender reputation negatively. Breaks DMARC.
Verify DNS record for syntax errors, check key rotation schedules. Refer to Yahoo DKIM failures.
DKIM temperror
Temporary issues, often due to DNS timeouts or server load. Can lead to deferred delivery.
Check DNS server health and network connectivity. Our guide on decoding DKIM temperror provides detailed solutions.
Views from the trenches
Best practices
Always use a consistent DKIM domain for all email campaigns that you want to share a unified sender reputation.
Implement DMARC alongside DKIM and SPF to gain visibility into your email authentication results.
Regularly check your DKIM records using multiple validation tools to catch any potential inconsistencies.
Common pitfalls
Assuming that a single validation tool's error report means your DKIM is universally failing.
Neglecting to monitor DMARC reports for real-world authentication outcomes from receiving mail servers.
Not rotating DKIM keys periodically, which can reduce security and impact reputation management over time.
Expert tips
If using multiple ESPs, consider using unique DKIM selectors for each to segment reputation and streamline troubleshooting.
Prioritize the authentication status reported in email headers and DMARC reports over strict online checker results.
When troubleshooting, verify DNS propagation times to ensure all services are seeing the most up-to-date record.
Expert view
Expert from Email Geeks says: Reputation is primarily tied to the signing domain (d= tag), not the selector, so focusing on the domain's reputation is key.
2019-08-28 - Email Geeks
Expert view
Expert from Email Geeks says: If you want all your email streams to share a unified reputation, it's appropriate to use the same DKIM signing domain across them.
2019-08-28 - Email Geeks
Ensuring robust email authentication
Using the same DKIM domain and selector for multiple email campaigns is not inherently problematic. The key takeaway is that your sender reputation is primarily tied to the DKIM signing domain (d= tag), and sharing it means sharing that reputation across all your email streams.
Discrepancies from validation tools often stem from minor syntax variations, DNS propagation delays, or differences in how strictly tools parse DKIM records compared to actual mail servers. By focusing on correct DNS record formatting, monitoring DMARC reports, and understanding how different systems interpret DKIM, you can ensure robust email authentication and excellent deliverability for all your campaigns.
SendGrid for transactional emails and another ESP for marketing, you might use a unique selector for each. This allows you to monitor and manage the reputation of each email stream independently. Twilio SendGrid documentation provides a good overview of setting up domain authentication, including custom selectors. Also, our article on multiple DKIM records for multiple ESPs can offer further insights.
