SPF=Fail, but the recipient’s mailbox has not quarantined or rejected the email- why?

When an email shows ‘SPF=fail’ but is not blocked by an antispam filter, it can be due to several reasons. Knowing and fixing the issue is important; otherwise, threat actors can exploit the security gap by sending fraudulent emails in your business’ name. 

SPF misalignment tolerance

Sometimes, SPF’s result is not considered a definitive indicator due to a few reasons-

Antispam systems settings

Antispam systems and tools are designed to consider several factors before tagging an email as ‘safe’ or ‘unsafe.’ Also, these systems are configured to be more lenient with SPF, which means they consider SPF’s result to be ‘one of the indicators’ and not ‘the most definitive indicator.’ If other indicators, like DKIM checks, DMARC checks, content analysis, sender reputation, rule-based filter, blocklist filter, etc., are favorable, the email won’t be quarantined or rejected by the recipient’s mailbox.

Soft fail policy

 The SPF record might be set to ~all (softfail) instead of -all (fail). A soft fail instructs the recipient’s mail server not to strictly disallow a suspicious email from entering the mailbox but instead suggests that such emails should be treated with caution. Antispam tools may allow these sorts of messages to pass through depending on the criteria. 

DMARC policy

If a domain has a DMARC policy in place that is set to p=none, the policy is only monitoring and not enforcing strict rejection. Even if the SPF check fails, the DMARC policy doesn’t require the email to be blocked.

DMARC also requires alignment between the SPF domain and the FROM address domain. If the alignment fails but the DMARC policy is set to p=none, the email is not blocked.

Sender reputation

Some antispam filters weigh the reputation of a sender domain or IP address more heavily than the SPF results. That’s why if a potentially fraudulent email is sent from a domain with a high reputation, it will likely pass through even with an SPF fail. This might not always be a key factor, but many tools still rely on it. 

Multiple SPF checks

There should only be one SPF record per domain. Having more than one SPF record is technically incorrect, but it happens in practice. So, if this is the case with your domain, then some systems may incorrectly parse or apply the SPF mechanism, especially if these records contradict each other. This leads to a failure in SPF validation without resulting in a block.

If you also have multiple SPF records corresponding to your domain, consider merging them into one. 

Fallback mechanisms

Fallback mechanisms are the additional checks and security measures used by antispam tools for a more nuanced evaluation of an email security and authenticity. These include monitoring factors like the volume of emails sent, the frequency of emails sent to specific recipients, and historical data on how recipients interact with these emails (e.g., high engagement rates).

Also, if the recipient whitelists a sender, the tool may disregard the SPF failure and let the email through. 

While not all the issues can be fixed at your end, try working around the ones emerging from your lack of attention or knowledge. For any help, contact us.