How to Troubleshoot SPF-Related Bounce-Backs

Understanding SPF Bounce-Backs

Before diving into SPF bounce-back troubleshooting, it’s important to understand how SPF failures usually present themselves in email delivery issues.

SPF failures rarely happen in isolation if you have DKIM and DMARC implemented properly.

DMARC requires either SPF or DKIM to align and authenticate the message to be accepted. If DMARC fails, it generally means both SPF and DKIM have failed to align and authenticate the email properly. In practice, you usually won’t see a bounce-back that says “SPF failed” alone without DKIM or DMARC issues.

1. Rare cases of SPF-only rejection do exist, but they are uncommon.
   Some recipient mail servers enforce strict SPF checks and might reject emails outright when SPF fails, without performing DKIM or DMARC checks. These setups are rare but possible, often in highly security-sensitive environments.
   
   
2. Some Security Email Gateways (SEGs) apply custom rules based on SPF results.
   Enterprises using advanced SEGs may reject messages solely based on SPF failure as part of their internal policy. This behavior varies widely depending on the recipient’s security configuration and policies.

Common SMTP bounce-back error codes

Since emails in general are not rejected only due to SPF, each SMTP bounce error indicates that there was an issue with SPF authentication as well. Below are examples of SMTP bounce-back errors:

4.X.X Persistent transient failures

The mail server encountered a temporary issue, often due to a timeout or delay on the recipient’s side. This isn’t caused by your configuration and typically resolves on its own after a short time. Some servers also use temporary errors to slow down untrusted or suspicious senders. These errors usually begin with codes like 421, 422, 450, 451, or 452, and may appear as "Temperror" in your DMARC aggregate reports.

5.X.X permanent error

The 5.x.x SMTP errors(e.g., 540 5.7.1) cover a wider range of bounce-back issues, and not all of those errors indicate a DMARC-related issue. It’s common for email service providers to add a status code (5.1.1) after the SMTP reply code (smtp;550) for additional categorization of the issue.
The permanent error codes that indicate DMARC failure due to SPF and DKIM authentication issues are:
550 5.7.1 - Your message was rejected due to example.com’s DMARC policy.

550 5.2.0 -  Message rejected due to DMARC

550 5.7.26 - <example@example.com>: DMARC Failure for domain

SPF failure troubleshooting steps 

If you receive a bounce-back message, there are some major steps you can go over to troubleshoot SPF issues:

• Verify the sending IP address: You can find the sending IP in the bounce-back message or by checking the original message headers of the failed email.
  This IP will either be a dedicated one or belong to an authorized sending service you use. Once verified as legitimate, ensure the IP is included in your SPF record.

• Check SPF Syntax and ensure it doesn’t exceed the SPF 10 lookup limit:
  You can use EasyDMARC’s SPF lookup tool to make sure your SPF is syntactically correct and doesn’t exceed the SPF’s 10 DNS lookup limitation.

• Make sure the SPF is properly configured for the sending source: Sending sources also differ in terms of their SPF capability and the configuration they support - SPF capable sources, SPF subdomain-capable sources, and SPF non-capable sources:
  The SPF-capable sources support SPF configuration on the root domain level. So all you need is to add the SPF mechanism of the sending source to your domain’s SPF record.
  The SPF subdomain-capable sources support SPF configuration on a subdomain level. You’ll typically create a CNAME record pointing to the provider’s SPF configuration or configure SPF directly on a subdomain.
  The SPF non-capable sources don’t support SPF alignment. SPF failures from these can be ignored, so you need to focus on DKIM configuration to ensure DMARC compliance.
  You can also refer to this article for more comprehensive guidance about different types of sources due to their SPF capabilities.

If you have any questions, don’t hesitate to reach out to EasyDMARC technical support.