How to troubleshoot SPF authentication issues with multiple ESPs, subdomains, and CNAMEs?
Matthew Whittaker
Co-founder & CTO, Suped
Published 4 Jul 2025
Updated 19 Aug 2025
7 min read
Managing email deliverability for an organization that uses multiple email service providers (ESPs), various subdomains, and complex CNAME configurations can quickly become a headache. Sender Policy Framework (SPF) is a critical email authentication protocol, but its implementation can be fraught with challenges when dealing with such diverse setups.
I've seen firsthand how easily SPF records can lead to authentication failures, especially when CNAMEs are involved in unexpected ways or when the DNS lookup limit is exceeded. These issues can result in legitimate emails landing in spam folders or being rejected outright, impacting communication and business operations.
This guide will walk you through the complexities of troubleshooting SPF authentication issues in environments with multiple ESPs, subdomains, and CNAMEs. We will cover common pitfalls, best practices, and systematic approaches to diagnose and resolve these challenging deliverability problems.
SPF is a DNS TXT record that specifies which mail servers are authorized to send email on behalf of your domain. Its primary goal is to prevent email spoofing. When a recipient mail server receives an email, it checks the sender's SPF record to verify that the sending IP address is on the list of approved senders for that domain.
A fundamental rule of SPF is that a domain or subdomain should only have one SPF record. Multiple SPF records for the same hostname will cause authentication to fail. Additionally, SPF records have a 10-DNS-lookup limit. Exceeding this limit will result in an SPF PermError, preventing legitimate emails from being authenticated.
For subdomains, the situation changes slightly. If you send email from marketing.yourdomain.com, it can (and often should) have its own distinct SPF record. This allows for greater control and isolation of your email streams, which can be beneficial for domain reputation management. However, issues arise when CNAME records are present on these subdomains.
The challenge of multiple ESPs
One of the most frequent challenges I encounter involves SPF with multiple ESPs. Each ESP will provide an include mechanism to add to your SPF record. For example, include:sendgrid.net for SendGrid or include:customeriomail.com for Customer.io. When you use multiple ESPs, you must combine all these includes into a single SPF record for your domain or subdomain. Having separate SPF records for the same domain is a common mistake that breaks authentication.
Another major hurdle is the 10-DNS-lookup limit. Each include, a, mx, and ptr mechanism in your SPF record counts as a lookup. If your SPF record involves too many nested lookups, it will fail validation, causing deliverability issues. This is particularly problematic for organizations that use many different vendors for their email sending.
To get around the lookup limit, many organizations turn to SPF flattening services or use a dedicated subdomain for each ESP. SPF flattening works by resolving all the included IPs and IP ranges into a single, comprehensive SPF record that doesn't exceed the 10-lookup limit. This is often the most practical solution for complex setups.
Single SPF record
Issue: Each domain or subdomain should only have one SPF TXT record. Multiple SPF records for the same hostname will cause authentication to fail, leading to non-delivery or spam placement.
Solution: Consolidate all authorized sending sources into a single SPF record. Merge the include mechanisms from all your ESPs into one v=spf1 record.
Subdomains and CNAME complexities
Subdomains are a popular way to segment email traffic, for example, using marketing.yourdomain.com for campaigns and transactional.yourdomain.com for transactional emails. Each subdomain can have its own SPF record, which helps manage the 10-lookup limit and isolates the reputation of different email streams. If one subdomain's email activity is flagged, it's less likely to impact the deliverability of emails sent from other subdomains.
However, CNAME records introduce a specific complexity with SPF. A CNAME record, by definition, aliases one domain name (or subdomain) to another. The critical rule is that if a domain (or subdomain) has a CNAME record, it should not have any other record types, including TXT records for SPF. This is a fundamental DNS principle. If you create a CNAME for inbound.yourdomain.com that points to yourdomain.com, any SPF lookup for inbound.yourdomain.com will follow the CNAME and evaluate the SPF record of yourdomain.com. This is well documented in email industry blogs and communities.
Root domain CNAME conflict
Issue: If your root domain (yourdomain.com) has a CNAME record, it cannot have an SPF TXT record directly on it. This is a common point of confusion when setting up SPF for custom tracking domains provided by ESPs that require CNAMEs.
Solution: Avoid CNAMEs at the root domain if you need an SPF record there. Instead, use an A record or ALIAS/ANAME record if your DNS provider supports it for root-level CNAME-like functionality.
This DNS behavior can lead to SPF authentication issues if you've configured a subdomain (e.g., inbound.segment.com) to CNAME to your root domain (segment.com) for purposes like HTTPS link tracking. In such a scenario, the SPF check for inbound.segment.com will actually use the SPF record of segment.com. If your ESP expects a specific SPF record on the subdomain, this setup will lead to validation failure. You can learn more about this by referring to discussions in technical forums.
Troubleshooting steps
When SPF authentication issues arise, a systematic troubleshooting approach is key. Start by checking your SPF record directly using a DNS lookup tool. Verify that there is only one TXT record starting with "v=spf1" for the specific domain or subdomain you are troubleshooting. If you find multiple records, you'll need to merge them into a single entry.
Next, examine the include mechanisms to ensure all your sending ESPs are listed and that the total number of DNS lookups does not exceed ten. If it does, consider SPF flattening or dedicated subdomains for each ESP. Pay close attention to any ptr mechanisms, as these are deprecated and can cause issues with some receivers.
Before: common issues
Multiple records: Having more than one SPF TXT record for a single domain, which immediately breaks authentication.
Lookup limit: Exceeding the 10-DNS-lookup limit due to numerous include mechanisms or complex nested lookups.
CNAME conflicts: Placing an SPF TXT record on a domain or subdomain that also has a CNAME record, particularly at the root.
After: solutions
Consolidation: Merge all valid include directives into a single SPF record for each sending domain or subdomain.
SPF flattening: Use a service that automatically resolves all included IPs into a single SPF record to stay within the lookup limit.
Subdomain strategy: Dedicate specific subdomains for each ESP, each with its own SPF record, preventing CNAME-related conflicts on the root domain.
For CNAME-related issues, verify that no TXT record (for SPF) exists on a domain or subdomain that also has a CNAME record pointing elsewhere. If a CNAME is necessary for link tracking or other purposes, you may need to ensure your emails are sent from a different subdomain that does not have a CNAME to the primary domain. If your DMARC reports show SPF failures, these reports are your best friends in identifying the exact cause of authentication issues.
Conclusion
SPF authentication issues can significantly hinder your email deliverability. Understanding the interplay between multiple ESPs, subdomains, and CNAME records is crucial for maintaining a healthy email program. By systematically checking for common misconfigurations like multiple SPF records, DNS lookup limit overages, and CNAME conflicts, you can proactively address and prevent most SPF authentication problems. Prioritizing correct SPF setup will contribute greatly to ensuring your legitimate emails reach the inbox reliably.
Always remember that DNS changes can take time to propagate, so patience is key during the troubleshooting process. Regularly monitoring your email authentication status and DMARC reports will provide ongoing insights into your SPF performance and help you catch issues early. A well-configured SPF record is a foundational element of strong email deliverability.
Views from the trenches
Best practices
Consolidate all ESPs into a single SPF record per domain/subdomain.
Use dedicated subdomains for different email streams and ESPs.
Implement SPF flattening to manage the 10-DNS-lookup limit.
Regularly monitor your DMARC reports for SPF authentication failures.
Test SPF record changes thoroughly using online tools before deployment.
Common pitfalls
Having multiple SPF TXT records for the same domain or subdomain.
Exceeding the 10-DNS-lookup limit, causing PermErrors.
Placing an SPF record on a domain/subdomain that also has a CNAME record.
Failing to update SPF records when adding or removing ESPs.
Misunderstanding how CNAMEs affect SPF resolution for subdomains.
Expert tips
When troubleshooting, check the actual DNS resolution, not just what your ESP's dashboard says.
Separate transactional and marketing emails onto different subdomains to protect your reputation.
If using a DMARC management service, ensure they are correctly handling SPF flattening.
Be cautious with CNAMEs at the root domain, as they conflict with other record types.
Always verify if a subdomain CNAMEs to the root, as this impacts which SPF record is checked.
Expert view
Expert from Email Geeks says they do not see a direct connection between HTTPS authentication and SPF authentication, as they are distinct concepts.
2019-06-25 - Email Geeks
Expert view
Expert from Email Geeks says that for multiple ESPs, using separate subdomains, one for each ESP, allows for custom SPF records for each, simplifying management.
