Summary
MXToolbox may display email authentication errors despite an ESP reporting success due to various factors: DNS/DNSSEC issues, MXToolbox using outdated information or different testing locations, incorrect SPF macro handling, syntax errors in SPF records, exceeding DNS lookup limits, DKIM key rotation problems, or ESP whitelisting. Consequently, MXToolbox results should be viewed as a snapshot in time and not the sole determinant of deliverability. To address bot unsubscribes, avoid single-click unsubscribes. Implement strategies like confirmation pages (with CAPTCHAs), 'List-Unsubscribe-Post' headers, double opt-in, honeypot fields, and monitor unsubscribe rates. Compliance with SPF, DKIM, and DMARC remains crucial for sender reputation and deliverability.
Key findings
- MXToolbox Inaccuracy: MXToolbox can be inaccurate due to DNS/DNSSEC, outdated data, location differences, SPF macro issues, and more.
- Authentication Passing: DKIM alignment or ESP whitelisting may result in emails passing authentication despite MXToolbox's flags.
- Root Cause: Bots: Single-click unsubscribe options are easily exploited by bots, leading to unintended unsubscriptions.
- Unsubscribe Methods: Confirmation webpages, CAPTCHAs, and 'List-Unsubscribe-Post' headers can prevent bot unsubscribes.
- Record Importance: Compliance with SPF, DKIM, and DMARC is vital for sender reputation and email delivery.
- Secondary Tool: MXToolbox is a secondary tool and should not be relied on as the only source of data.
Key considerations
- Check DNS: Investigate potential DNS or DNSSEC problems if MXToolbox reports errors.
- Verify Authentication: Manually inspect email headers and DNS records to verify authentication results.
- Implement Unsub Confirmation: Implement a confirmation webpage for unsubscribes, ideally with a CAPTCHA or honeypot.
- Use ESP Info: Prioritize the testing provided by the ESP.
- Authentication Check: Verify SPF, DKIM and DMARC configurations for optimal deliverability.
- Monitor Rates: Monitor unsubscribe rates for suspicious patterns.
What email marketers say
9 marketer opinions
Discrepancies between MXToolbox and ESP results for email authentication can stem from several factors. DNS propagation delays, MXToolbox using outdated information or different testing locations, ESP whitelisting, and the complexity of authentication setups all contribute to potential inaccuracies. MXToolbox results should be viewed as a snapshot in time and not the sole indicator of deliverability. Bot unsubscribes, on the other hand, are addressed through implementing double opt-in, CAPTCHAs, unsubscribe confirmation pages, and monitoring unsubscribe rates for suspicious activity.
Key opinions
- DNS Issues: DNS propagation delays and different testing locations can cause MXToolbox to report different results than ESPs.
- MXToolbox Limitations: MXToolbox provides a snapshot in time and may not reflect real-world deliverability.
- ESP Whitelisting: ESPs might whitelist your domain, bypassing DMARC policies and leading to differing results.
- Bot Unsubscribes: Bot unsubscribes are often due to bots clicking unsubscribe links.
- Authentication Complexity: Email authentication is complex, and MXToolbox results should be viewed alongside other testing and deliverability analysis.
Key considerations
- Verify DNS: Verify DNS settings using multiple tools and allow sufficient time for propagation.
- Monitor DMARC: Monitor DMARC reports to identify authentication failures.
- Implement Bot Prevention: Implement double opt-in, CAPTCHAs, or unsubscribe confirmation pages to prevent bot unsubscribes.
- Use Multiple Tools: Don't rely solely on MXToolbox; use it in conjunction with other testing methods.
- Check ESP Whitelisting: Consider whether your ESP is whitelisting your domain and how that affects reported authentication results.
Marketer view
Email marketer from Mailjet shares that a DMARC policy of 'reject' or 'quarantine' can cause MXToolbox to flag issues if SPF or DKIM checks fail, even if the ESP reports successful delivery to some recipients. They advises monitoring DMARC reports to identify authentication failures and adjust SPF/DKIM settings or DMARC policy accordingly.
10 Jun 2024 - Mailjet
Marketer view
Email marketer from Superuser explains that the ESP might be whitelisting your domain and this is the reason why the results are different. For example, the DMARC policy will be ignored if you are whitelisted. This explains the differences in email authentication results.
10 Jun 2025 - Superuser
What the experts say
6 expert opinions
MXToolbox may report email authentication errors despite ESP success due to DNS/DNSSEC issues, incorrect SPF macro handling, or simply outdated information from the last check. A core issue is often MXToolbox checking aspects not directly related to the actual mail flow. For bot unsubscribes, the key is moving away from single-click unsubs in emails. Using a webpage with a confirmation button (potentially with CAPTCHA) is the recommended fix. Ensuring compliance with SPF, DKIM, and DMARC is essential for overall deliverability.
Key opinions
- MXToolbox Reliability: MXToolbox can be unreliable due to DNS/DNSSEC problems, SPF macro misinterpretation, and outdated information.
- Authentication Passing: DKIM alignment might mean email is passing authentication despite MXToolbox's flags.
- Bot Unsubscribes Cause: Single-click unsubscribe options in emails are easily exploited by bots.
- Unsubscribe Fix: Using a confirmation webpage (with CAPTCHA) can prevent bot unsubscribes.
- Authentication Importance: Compliance with SPF, DKIM, and DMARC is important for sender reputation.
Key considerations
- Check DNS/DNSSEC: Investigate potential DNS or DNSSEC problems if MXToolbox reports errors.
- Test Authentication: Manually inspect email headers and DNS records to verify authentication results.
- Implement Unsubscribe Page: Implement a confirmation webpage for unsubscribes, including a CAPTCHA.
- Use Multiple Tools: Don't rely solely on MXToolbox; confirm results with your ESP and other tools.
- Ensure Authentication Compliance: Verify SPF, DKIM and DMARC configurations for optimal deliverability.
Expert view
Expert from Email Geeks explains that MXToolbox is generally reliable but the issues reported sound like either messed up DNS or DNSSEC problems.
18 Feb 2022 - Email Geeks
Expert view
Expert from Spamresource explains that often times the results differ due to the timing. MXToolbox can provide results based on its last check. It recommends using other tools such as the ESP as they are more reliable.
15 Feb 2022 - Spamresource
What the documentation says
6 technical articles
MXToolbox showing email authentication errors despite ESP success often relates to misconfigured SPF or DKIM records. SPF issues can stem from syntax errors, exceeding DNS lookup limits, or improper 'include:' statements, while DKIM failures may be due to key rotation problems or incorrect selector settings. Bot unsubscribes are addressed by implementing strategies beyond one-click unsubscription, such as using a 'List-Unsubscribe-Post' header, confirmation pages, or 'honeypot' fields. DMARC is designed to prevent email spoofing.
Key findings
- SPF Errors: SPF errors in MXToolbox can arise from syntax mistakes, exceeding DNS lookup limits, or incorrect 'include:' usage.
- DKIM Failures: DKIM failures, despite ESP success, may result from key rotation issues, incorrect selector settings, or DNS caching problems.
- Bot Unsubscribe Abuse: One-click unsubscription can be exploited by bots.
- SPF exists Mechanism: The SPF exists mechanism confirms the existence of a domain name for authentication.
- DMARC Purpose: DMARC protects against email spoofing and unauthorized domain use.
- Honeypot Fields: A 'honeypot' field, invisible to humans, can identify and block bot submissions.
Key considerations
- Validate SPF: Use SPF record validation tools to identify and correct any errors.
- Check DKIM: Verify DKIM key validity, selector configuration, and DNS records using diagnostic tools.
- Use List-Unsubscribe-Post: Implement a 'List-Unsubscribe-Post' header to prevent bot exploitation of one-click unsubscription.
- Implement Honeypots: Consider adding a honeypot field to your email forms to detect and block bots.
- Understand DMARC: Ensure you have a working DMARC policy to protect your domain.
Technical article
Documentation from DigitalOcean suggests one solution to bot attacks is implementing a 'honeypot field'. This is a hidden form field that bots will fill out but humans will not see. If the honeypot field is filled, the request is discarded.
27 Jun 2022 - DigitalOcean
Technical article
Documentation from Microsoft Learn explains DKIM failures even when the ESP reports success may result from key rotation issues (new keys not properly propagated), incorrect selector settings in DNS, or DNS caching problems. The documentation suggests checking the DKIM key validity, selector configuration, and DNS records using diagnostic tools.
14 Oct 2021 - Microsoft Learn
Can spam filters trigger email unsubscribes and how to prevent it?
How can I detect and segment bot clicks in email campaigns?
How can I prevent bot signups on my email newsletter form?
How can I troubleshoot DMARC failures and identify the cause of authentication issues?
How do I troubleshoot and fix SPF and DMARC settings for email deliverability issues?
How do SPF, DKIM, and DMARC email authentication standards work?
