What is the impact of exceeding the SPF DNS lookup limit?

Sender Policy Framework (SPF) is a crucial email authentication standard that helps protect your domain from being used for phishing and spoofing. It works by specifying which mail servers are authorized to send email on behalf of your domain. When a receiving mail server gets an email, it checks the sender's SPF record to verify the source. This check involves performing DNS lookups. However, there's a strict limit: only 10 DNS lookups are allowed per SPF check.

This limit is in place to prevent server strain and potential denial-of-service (DoS) attacks that could arise from overly complex SPF records. But as businesses use more and more third-party services to send email (like marketing platforms, CRMs, and helpdesks), it's become increasingly easy to accidentally exceed this limit. The consequences of doing so can be severe for your email program.

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When a receiving mail server tries to validate an SPF record that requires more than 10 DNS lookups, the process immediately stops and returns a permanent error, often seen as PermError: Too many DNS lookups. This isn't just a warning; it's a hard failure. The server essentially gives up on trying to figure out if the email is legitimate because the instructions you provided are too complicated.

Email Deliverability and Automation Expert says:

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Exceeding the 10 DNS Lookup limit results in a 'PermError SPF permanent error: too many DNS lookups.' This invalidates your SPF record, leading to authentication failure. Consequently, legitimate emails may be flagged as spam or rejected.

This PermError effectively invalidates your SPF record for that specific email check. Even if the email was sent from a perfectly legitimate source listed in your SPF record, the receiver can't confirm it because it couldn't complete the check. The result is an immediate SPF failure.

An SPF PermError is not a silent failure. It has direct and damaging consequences for your ability to reach the inbox and maintain your sender reputation. The key impacts include:

• Email Rejection or Spam Placement: Because your SPF record is invalid, receiving servers lose a key signal for trusting your emails. This makes it much more likely that your messages will be sent directly to the spam folder or rejected entirely. As DuoCircle notes, this directly affects email deliverability.
• DMARC Failure: If you're using DMARC (and you should be), SPF is likely one of your authentication methods. A PermError causes SPF to fail, which in turn can cause your DMARC check to fail. If your DMARC policy is set to p=quarantine or p=reject, your legitimate emails will be quarantined or rejected, even if DKIM passes.
• Damaged Sender Reputation: Consistent SPF failures signal to mailbox providers like Gmail and Outlook that your domain has poor email hygiene. This can damage your sender reputation, making it harder to reach the inbox even after you fix the issue.
• Increased Vulnerability to Spoofing: An invalid SPF record means you lose a layer of protection against phishing and spoofing. Cybercriminals can more easily impersonate your domain to send malicious emails, as receiving servers can't use SPF to validate the sender.

Sendmarc says:

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Exceeding this limit leads to SPF failures, which impact deliverability and open the door to email-based threats.

The most common cause of exceeding the 10-lookup limit is the use of the include: mechanism. Each include: statement in your SPF record adds one DNS lookup. However, if the included SPF record itself contains more include: statements (a nested include), those also count toward your total. It's a chain reaction that can quickly push you over the limit.

To prevent this, you should regularly audit your SPF record. Remove any services you no longer use. If you are still over the limit, you may need to implement a technique called SPF flattening. This involves replacing include mechanisms with the specific IP addresses they point to, which reduces the number of lookups required. The main takeaway is that managing your SPF record is not a one-time setup; it requires ongoing attention to ensure your emails continue to be delivered and your domain remains secure.