Key Takeaways
- “Best Guess Pass” is an informal term in email authentication reports that indicates messages passed SPF and/or DKIM checks, but no DMARC record exists.
- It is not defined in the official DMARC specification (RFC 7489).
- Microsoft Exchange Online Protection uses it to show alignment with the sender’s domain.
- Gmail and some providers apply “best guess” mainly to SPF by synthesizing a missing record.
- The presence of “Best Guess Pass” highlights a missing DMARC record, creating a security gap.
- Publishing a DMARC record with a clear policy (none, quarantine, or reject) prevents this issue and strengthens domain protection.
“Best Guess Pass” is not an official DMARC result and does not appear in the DMARC specification (RFC 7489). The term comes from how some receiving mail servers, such as Microsoft Exchange Online Protection, handle emails that pass SPF or DKIM checks but lack a DMARC record. In these cases, the server interprets the authentication as valid and labels it a “best guess pass,” meaning that if DMARC were configured, the email would have passed. While the phrase “best guess” is more commonly linked to SPF, its appearance in DMARC reports points to a critical problem: the absence of a DMARC policy. Recognizing this gap is essential for improving email security.
Do not confuse:
- Microsoft → labels DMARC reports with dmarc=bestguesspass.
- Gmail → synthesizes missing SPF records (not DMARC).
How DMARC Works in Normal Scenarios
To understand the problem with a “best guess,” let’s quickly recap how DMARC is supposed to work. DMARC relies on two other email authentication protocols, SPF and DKIM:
- SPF: This is a DNS record that lists the IP addresses that have the right to send email on behalf of your domain. You can use a free SPF generator to create your SPF record if you don’t have one. If you do have one but aren’t sure about its accuracy, use an SPF checker.
- DKIM: This protocol provides a cryptographic signature that checks whether or not a message has been manipulated in transit. A DKIM record generator and DKIM record checker can help you if you have struggles with setting up DKIM.
DMARC then checks that at least one of these methods (SPF or DKIM) not only passes but also aligns with the domain in the “From” address (i.e., the one the recipient sees).
Based on this check, a DMARC-compliant receiver produces one of two official outcomes (but reporting details can differ):
- Pass: The email is authenticated and aligned.
- Fail: The email is not authenticated or aligned.
Your DMARC policy then instructs the receiver on how to deal with emails that fail the check:
- p=none: Monitor only. Deliver the email. Note that, while emails are still delivered, aggregate reporting begins, which is the main benefit of p=none.
- p=quarantine: Send the email to the spam or junk folder.
- p=reject: Block the email entirely.
What Causes a “DMARC Best Guess Pass”?
A “Best Guess Pass” result typically appears when no DMARC record exists and the underlying SPF/DKIM checks pass.
Here’s the typical scenario:
- You or another authorized party sends an email from your domain.
- Your domain has valid SPF and/or DKIM records.
- The receiving server checks SPF/DKIM, and they pass with proper alignment.
- The receiver then looks for a DMARC record to see what policy to apply.
- It finds no DMARC record.
- Because the underlying authentication passed, the receiver makes a “best guess” and lets the email through without taking any DMARC action. It logs this as something like dmarc=bestguesspass.
This is a fallback mechanism. The provider is trying to avoid blocking potentially legitimate email just because a DMARC record is missing, but it highlights a significant configuration oversight.
Why is “Best Guess Pass” a Problem?
Relying on a “Best Guess Pass” is risky and undermines the purpose of DMARC.
It Creates Confusion
This unofficial status makes DMARC reports harder to interpret. You might think your domain is protected when it isn’t.
It Weakens Security Visibility
A “Best Guess Pass” tells you nothing about fraudulent emails. Since you don’t have a DMARC policy, you won’t receive reports on spoofing attempts, leaving you blind to attacks targeting your domain.
It Allows Phishing and Spoofing
Without a p=quarantine or p=reject policy, you have no defense. Scammers can still spoof your domain, and receiving servers that don’t perform this “best guess” check (which is most of them) will have no instructions to block the fraudulent emails.
How to Fix “DMARC Best Guess Pass” Issues
The fix is simple in concept: publish a DMARC record for your domain. This will help avoid guesses and tell the world exactly what to do with your email.
1. Have the Right SPF and DKIM Setup
Before you create a DMARC record, make sure that your SPF and DKIM records are correctly configured. They should include all legitimate sending services.
2. Verify Domain Alignment
Make sure the domain used for SPF (the Return-Path domain) and/or the domain in the DKIM signature (the d= tag) aligns with the “From” address domain.
3. Publish a DMARC Record
Start with a monitoring policy (p=none). This allows you to gather data without affecting your email deliverability. A basic starting record looks like this: v=DMARC1; p=none; rua=mailto:your-dmarc-reports@example.com;
- Place this TXT record at _dmarc.yourdomain.com.
4. Use a DMARC Reporting Platform
Raw DMARC reports are XML files, and they are quite difficult to read. A monitoring platform will turn these reports into human-readable dashboards. It will give you clear insights into who is sending emails from your domain.
Best Practices to Prevent False Pass Results
Audit DNS Records
Always check your SPF, DKIM, and DMARC records to ensure they are accurate and up-to-date.
Monitor DMARC Reports Daily
Keep a close eye on your DMARC reports to detect any new sending sources or potential authentication failures.
Implement a Stricter Policy
Once you are confident that all your legitimate emails are passing DMARC checks, you can now gradually move to a stricter policy like p=quarantine and eventually p=reject. This will help you actively block fraudulent emails.
Train Teams
Train your IT and security teams so that they know how to interpret DMARC data and respond to potential threats.
Summing Up
“Best Guess Pass” is not a sign of secure email; it’s a warning sign. It means that your domain’s email security is incomplete and relies on the non-standard behavior of a few mailbox providers. You must move beyond guesswork and set up DMARC to take control of your domain’s reputation and security.
Our expert team at PowerDMARC can help. We take care of everything DMARC-related so you can communicate with certainty, not confusion. Get in touch today!
Frequently Asked Questions
Why do I see “Best Guess Pass” in my reports?
You are likely seeing this result in reports from Microsoft 365 or Exchange. It means the sending domain has SPF/DKIM set up, but doesn’t have a DMARC record. The system is noting that the email would have passed DMARC if a policy existed.
Is “Best Guess Pass” a security risk?
Yes. It means a DMARC enforcement policy (quarantine or reject) is absent. Without enforcement (quarantine/reject), you can’t instruct receivers to block or divert unauthorized emails.
How can I stop “Best Guess Pass” from appearing in reports?
The sending domain owner must publish a valid DMARC record in their DNS. If it’s your domain, follow the steps in section 5.
Does “Best Guess Pass” mean my emails are secure?
No. It means your domain lacks a critical layer of security. Your emails can’t be secure if you don’t have a properly configured DMARC record.
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