What should I do if an unexpected IP address appears in Google Postmaster Tools?
Michael Ko
Co-founder & CEO, Suped
Published 11 May 2025
Updated 16 Aug 2025
7 min read
Discovering an unfamiliar IP address in your Google Postmaster Tools (GPT) dashboard can be unsettling. It suggests that emails appearing to be from your domain are being sent from a source you don't immediately recognize, impacting your IP reputation. However, it's not always a cause for panic. Often, there's a logical explanation, and identifying it requires a systematic approach.
My immediate reaction when I see this is to investigate the source. The key is to determine if the IP is legitimate, perhaps from a third-party service, or if it indicates unauthorized sending. This guide outlines the steps I take to identify the source of unexpected IPs and how to address the issue effectively.
Pinpointing the source of the unexpected IP
The first step is always to verify if the IP address genuinely belongs to a service you use, even if you don't directly manage it. Many companies utilize various email service providers (ESPs) or transactional email services for different sending purposes. These services might use a range of IP addresses, some of which you might not be immediately familiar with.
Start by compiling a comprehensive list of all third-party services that send email on behalf of your domain. This includes marketing automation platforms, CRM systems, customer support tools, and even internal corporate applications. Once you have the IP, a reverse DNS lookup can often reveal the true owner, providing a strong hint.
If a reverse DNS lookup on the IP points to a known cloud provider like Amazon Web Services (AWS), or an ESP like ActiveCampaign, it's a good sign that it might be legitimate, even if previously unnoticed. You can use tools like ARIN's RDAP or WHOIS to perform these lookups.
Performing reverse DNS lookup and WHOIS querybash
dig -x 192.0.2.1
whois 192.0.2.1
Common culprits behind unfamiliar IPs
One common reason for unexpected IPs to appear is mail forwarding. When an email is forwarded from one inbox to another, especially if the original DKIM signature is preserved, Google Postmaster Tools might attribute the sending IP of the forwarding server to your domain. This is not necessarily malicious but can certainly be confusing.
Another possibility involves internal corporate systems or test environments. Sometimes, an internal application or a developer's test script might inadvertently send emails using your domain without being explicitly listed in your SPF record or monitored. These internal sends, if routed through an unlisted IP, can show up in Postmaster Tools.
It's also worth noting that shared IP environments can contribute to this. If you are using a shared IP address space from an ESP, other senders' activity on those IPs can sometimes influence your reported reputation or show up in your data, even if their mail isn't directly from your domain. While less common for unexpected IPs, it's a factor in the broader context of IP reputation management.
Understanding mail forwarding impact
DKIM preservation: Forwarding services that don't alter the DKIM signature can make the forwarded email appear to originate from your domain's original sender.
IP attribution:GPT may incorrectly associate the forwarding server's IP with your domain, leading to unexpected entries.
Unlocking insights with DMARC reports
For me, the most robust way to diagnose unexpected IPs is by analyzing DMARC reports. If you have DMARC implemented, the aggregate (RUA) reports provide granular data on all IPs sending email purporting to be from your domain, along with their SPF and DKIM authentication results. This data is invaluable for pinpointing unauthorized or unexpected senders.
Even if you only receive raw XML DMARC reports, these files contain the necessary information. You can parse them manually or use a DMARC analysis tool (often available as part of DMARC monitoring services) to make the data more readable. Look for the specific IP address that appeared in Google Postmaster Tools within these reports. The reports will show if emails from that IP passed or failed SPF and DKIM authentication for your domain.
If the unexpected IP is failing DMARC, it's a strong indicator of a misconfiguration or even a malicious actor. If it's passing, it means the IP is authorized to send on your behalf, but you may not have been aware of it. This highlights the importance of having a robust DMARC implementation and a system for regularly reviewing your reports.
A key piece of information in the DMARC aggregate reports is the <header_from> field, which indicates the domain shown to the end-user. Cross-referencing this with the sending IP address allows you to confirm if the IP is sending mail from your primary domain or a subdomain you might not be tracking closely. This helps identify shadow IT or services that were set up without full awareness of your email infrastructure team.
Proactive steps to prevent future surprises
Once you've identified the source of the unexpected IP address, you can take steps to manage its impact. If it's a legitimate sender, ensure that its IP range is correctly included in your SPF record. This signals to receiving mail servers that these IPs are authorized to send email on your domain's behalf, helping to improve your deliverability and prevent blacklisting (or blocklisting) issues.
Regularly review your SPF record to ensure it's up-to-date and includes all your legitimate sending sources. An outdated SPF record can lead to legitimate emails failing authentication, harming your domain and IP reputation. Similarly, ensure your DKIM records are correctly configured for all your sending services.
If the IP is unauthorized, and you've confirmed it's not a misconfiguration, it could indicate spoofing or phishing attempts. In such cases, enforcing a strict DMARC policy (e.g., p=quarantine or p=reject) helps protect your domain by instructing receiving mail servers how to handle emails that fail DMARC authentication. This is crucial for preventing abuse of your brand and maintaining a healthy email ecosystem.
Recommended SPF record structure
Ensure your SPF record lists all authorized sending sources. Here's a basic example, but always consult your ESPs for their specific includes:
The -all mechanism is important as it indicates that only the listed IPs are authorized, and any others should be rejected (or hard failed).
Views from the trenches
Best practices
Actively use DMARC reports to monitor all IPs sending on behalf of your domain, including those you don't recognize immediately.
Maintain a comprehensive list of all your legitimate sending services, ensuring their IPs are correctly authorized in SPF.
Regularly review DNS records for SPF and DKIM to catch any misconfigurations or unauthorized additions promptly.
Implement a DMARC policy (even p=none initially) to gain visibility into your email ecosystem and identify unknown senders.
Common pitfalls
Ignoring unexpected IPs in Postmaster Tools, assuming they are false positives without proper investigation.
Failing to update SPF records when adding new sending services or changing existing ones, leading to authentication failures.
Not having a DMARC record in place, which means you lack visibility into who is sending email purporting to be from your domain.
Overlooking mail forwarding as a common source of unexpected IPs in your reports, leading to unnecessary concern.
Expert tips
Perform reverse DNS lookups (PTR records) and WHOIS queries on any unexpected IPs to identify their owners. This often reveals if it's a legitimate ESP or cloud provider.
Check your DMARC aggregate reports for the unexpected IP. These reports show if the email passed SPF and DKIM authentication for your domain, which is crucial for diagnosis.
Be aware that mail forwarding can cause IPs of intermediate servers to appear in Postmaster Tools if the DKIM signature remains intact.
Ensure your SPF record is always up-to-date with all authorized sending IPs and includes a strict failure mechanism like -all.
Marketer view
Marketer from Email Geeks says to confirm if the IP belongs to one of the client's email service providers or other third-party sending platforms.
2021-04-06 - Email Geeks
Expert view
Expert from Email Geeks says that the presence of an IP in Postmaster Tools indicates that mail authenticated as the domain.
2021-04-06 - Email Geeks
Maintaining a clear sending picture
Dealing with an unexpected IP address in Google Postmaster Tools requires a methodical approach. It begins with identifying the IP's owner, whether through reverse lookups or DMARC reports, and then determining if the sending is legitimate or unauthorized. Most often, these IPs turn out to be from a known, albeit sometimes overlooked, sending service or a result of mail forwarding.
Maintaining a clean and accurate SPF record and consistently monitoring your Google Postmaster Tools and DMARC reports are essential practices. These steps not only help in quickly addressing anomalies like unexpected IPs but also contribute significantly to your overall email deliverability and sender reputation.
Amazon Web Services (AWS), or an ESP like 
ActiveCampaign, it's a good sign that it might be legitimate, even if previously unnoticed. You can use tools like ARIN's RDAP or WHOIS to perform these lookups.
