What should I do about a weird SPF domain/IP sending from my client's domain?
Matthew Whittaker
Co-founder & CTO, Suped
Published 6 Jun 2025
Updated 16 Aug 2025
6 min read
Discovering an unfamiliar SPF domain or IP address sending emails from your client's domain can be quite alarming. It immediately raises questions about unauthorized activity and potential security breaches. This kind of unexpected entry in your email logs, whether it's an IP address you do not recognize or a domain name that seems completely out of place, demands immediate attention.
Such incidents can severely impact your client's sender reputation and email deliverability. If unaddressed, these unauthorized sending sources might lead to your client's emails being flagged as spam, ending up in junk folders, or even getting blocklisted (blacklisted) by major email providers. It is crucial to investigate these anomalies thoroughly to safeguard email integrity and ensure messages reach their intended inboxes.
The first step is to accurately identify the source of the unexpected sending activity. This involves reviewing your email logs, DMARC reports, and any other available deliverability monitoring tools. Look for specific IP addresses, reverse DNS entries, or domains associated with the suspicious email traffic. Sometimes, an unexpected IP in Google Postmaster Tools can be the first indicator.
An SPF record, or Sender Policy Framework record, is a type of DNS TXT record that identifies which mail servers are authorized to send email on behalf of your domain. When a receiving server gets an email, it checks the SPF record of the sending domain to verify that the email originated from an authorized server. If the sending IP address is not listed in the SPF record, it can indicate a spoofing attempt or a misconfiguration. You can learn more about what SPF is in our detailed guide.
Check your client's existing SPF record to see if this weird domain or IP is somehow included. Sometimes, a legitimate third-party sender might have been added without proper communication, or a misconfigured record could be pulling in unexpected includes. You can use an online tool to parse the SPF record and check its validity and includes. For a solid understanding of how SPF records are structured and validated, refer to this Cloudflare article on SPF records.
Common causes for unexpected SPF results
Forgotten senders: Past email service providers (ESPs) or marketing platforms that were never removed from the SPF record.
Typographical errors: Mistakes in typing IP addresses or domain names when updating the record.
Unauthorized subdomains: Someone sending from an unmanaged subdomain that inherits the main domain's SPF record.
DNS lookup limits: Exceeding the 10 DNS lookup limit in an SPF record, which can lead to SPF PermError failures.
Leveraging DMARC for domain protection
While SPF helps identify authorized senders, DMARC (Domain-based Message Authentication, Reporting, and Conformance) builds upon SPF and DKIM to provide a comprehensive email authentication framework. DMARC allows domain owners to tell receiving email servers what to do with messages that fail SPF or DKIM authentication, and it provides reports on email authentication results. This is crucial for protecting your client's domain from email spoofing attempts.
DMARC policies are defined in your DNS as a TXT record, similar to SPF. There are three main policy options: p=none, p=quarantine, and p=reject. Understanding the differences is key to implementing effective protection for your client's domain.
Passive monitoring (p=none)
This policy allows you to collect DMARC reports without affecting email delivery. It is ideal for initial setup to understand your email ecosystem and identify all legitimate sending sources.
Active enforcement (p=quarantine / p=reject)
These policies instruct receiving servers to either move failed emails to the spam folder (quarantine) or reject them outright. This is where you actively stop unauthorized emails from reaching inboxes. Moving from p=none to p=quarantine or reject should be done carefully, as discussed in our guide on transitioning your DMARC policy.
If your DMARC reports show significant legitimate email traffic failing SPF authentication, it points to a problem with your SPF record itself or your email setup. Conversely, if you see high volumes of traffic from the 'weird' domain/IP failing SPF and DMARC, it strongly indicates spoofing. This data is critical for making informed decisions on how to proceed.
Responding to confirmed unauthorized sending
Once you have analyzed the DMARC reports and confirmed that the weird SPF domain/IP is indeed an unauthorized sender, taking action is necessary. The primary goal is to prevent these unauthorized emails from reaching their recipients' inboxes and to protect your client's domain reputation. This often involves tightening your DMARC policy.
If your DMARC policy is currently set to p=none, consider moving to p=quarantine or even p=reject. This instructs receiving mail servers to treat emails that fail SPF or DKIM alignment based on your policy. While many receiving servers will eventually block obvious spam regardless of policy, an explicit DMARC enforcement policy provides a clear directive and strengthens your domain's authentication posture.
Best practice for DMARC enforcement
If you are confident that your legitimate email traffic is properly authenticated with SPF and DKIM, do not hesitate to move to a DMARC policy of p=quarantine or p=reject. This proactive measure helps email providers like Google and Yahoo better understand how to handle unauthenticated mail originating from your domain. It demonstrates that you are actively managing your email security, which contributes positively to your domain reputation. Most ESPs are sophisticated enough to filter spam, but explicit policies help them make the right call quicker.
Remember to continuously monitor your DMARC reports after making changes. These reports provide invaluable feedback on your email authentication status, helping you identify any new unauthorized senders or unexpected issues. Consistent monitoring ensures that your email security measures remain effective over time.
Ongoing vigilance and best practices
Email sending infrastructure is not static. As your client adds new services or changes existing ones, their SPF and DKIM records will need updates. Proactive management of these DNS records is essential to prevent future instances of unexpected SPF domains or IPs. Regularly audit all services authorized to send emails on behalf of your client's domain to ensure they are all properly included in SPF and DKIM.
Maintaining a clean and accurate SPF record is a cornerstone of good email deliverability. Avoid common pitfalls like exceeding the 10-lookup limit or using the dangerous +all mechanism, which allows anyone to send email on your behalf. Proper configuration helps ensure your legitimate emails avoid spam folders and your domain remains trustworthy, avoiding email deliverability issues.
Mechanism
Description
Impact
ip4/ip6
Authorizes specific IPv4 or IPv6 addresses.
Direct authorization, straightforward.
a/mx
Authorizes A records or MX records of the domain.
Useful for servers directly hosting email for the domain.
include
Includes the SPF record of another domain, e.g., include:sendgrid.net.
Commonly used for ESPs and third-party senders, contributes to DNS lookup limit.
all
Defines the default rule for senders not explicitly matched. Can be -all (fail), ~all (softfail), or +all (pass).
Crucial for defining policy. +all should be avoided.
Regularly auditing your SPF records, verifying all legitimate sending sources, and implementing a strong DMARC policy are paramount for protecting your client's domain. Staying vigilant will help you detect and mitigate unauthorized sending activity, ensuring your client's email program remains healthy and effective.
Conclusion
Dealing with unexpected SPF domains or IPs sending from your client's domain can be a complex issue, but it is manageable with a systematic approach. By carefully investigating the source, understanding the interplay of SPF and DMARC, and taking proactive steps to tighten your authentication policies, you can effectively combat spoofing and maintain a robust email infrastructure.
The key is continuous monitoring and adaptation. Email security is an ongoing process, not a one-time fix. Regularly reviewing your email authentication reports and staying informed about best practices will ensure your client's domain remains protected and their messages consistently reach the inbox.
Views from the trenches
Best practices
Always maintain an up-to-date inventory of all email sending services and platforms your client uses, ensuring each is authorized in the SPF record.
Implement DMARC with a p=none policy initially to gather comprehensive reports and identify all legitimate sending sources before moving to enforcement.
Regularly review your DMARC aggregate and forensic reports to detect unauthorized sending attempts and monitor authentication rates.
Ensure your SPF record adheres to the 10 DNS lookup limit to prevent SPF PermError failures and maintain optimal email deliverability.
Common pitfalls
Forgetting to update SPF records when migrating to new email service providers or adding new marketing platforms, leading to authentication failures.
Setting a DMARC policy to p=quarantine or p=reject too quickly without thoroughly analyzing DMARC reports, which can cause legitimate emails to be blocked.
Neglecting to monitor DMARC reports, missing critical insights into spoofing attempts or authentication issues that impact deliverability.
Using a broad SPF mechanism like +all, which effectively allows any sender to send emails on behalf of your domain, creating a severe security vulnerability.
Expert tips
Consider a DMARC-as-a-service provider to simplify report analysis and gain better visibility into your email ecosystem.
Educate your clients on the importance of email authentication and the risks of unauthorized sending.
Implement a DMARC policy even for domains that do not send email to prevent them from being spoofed.
Regularly test your email authentication setup using online tools to ensure everything is configured correctly and functioning as expected.
Marketer view
Marketer from Email Geeks says they found an unfamiliar SPF domain/IP sending from their client's domain and was unsure why it was showing up.
2023-01-15 - Email Geeks
Marketer view
Marketer from Email Geeks recommends using an online SPF checker tool if you are not familiar with DNS local lookups to understand the SPF entry.
Google and 
Yahoo better understand how to handle unauthenticated mail originating from your domain. It demonstrates that you are actively managing your email security, which contributes positively to your domain reputation. Most ESPs are sophisticated enough to filter spam, but explicit policies help them make the right call quicker.
include:sendgrid.net.
