What causes unexpected IP addresses to appear on Senderscore for dedicated IPs?
Matthew Whittaker
Co-founder & CTO, Suped
Published 22 Jul 2025
Updated 19 Aug 2025
6 min read
Discovering an unexpected IP address on SenderScore.org can be quite alarming, especially when you are certain your emails are only being sent from a dedicated IP address through a single Email Service Provider (ESP). This situation often leads to confusion, as the IP may not appear when you search for it individually, only as part of your domain's sending activity.
While the initial reaction might be concern about unauthorized sending, there are several common and often innocuous reasons why these unfamiliar IP addresses might surface. Understanding these causes is crucial for accurately interpreting your sender reputation data and maintaining optimal email deliverability.
Understanding SenderScore and dedicated IPs
SenderScore is a widely used metric that assesses the trustworthiness of an IP address on a scale of 0-100. A higher score indicates better email deliverability prospects. For businesses using dedicated IP addresses, the expectation is that their sending reputation is solely based on their own sending practices. This offers greater control compared to shared IP addresses, where reputation is influenced by all senders using that same IP. However, even with dedicated IPs, other factors can introduce unexpected complexities.
What is SenderScore?
SenderScore is a reputation system that evaluates an IP's sending behavior and assign a numerical score. This score is a key indicator for mailbox providers to determine whether to accept or reject incoming mail, or if it should be placed in the spam folder.
The value of dedicated IPs
Dedicated IPs provide complete control over your sending reputation. Your SenderScore reflects only your sending practices, making it easier to monitor and manage your deliverability. This is why it can be confusing when other IPs appear.
When you see an unexpected IP, it's essential to understand that reputation services like SenderScore (which is powered by ReturnPath) collect data from various sources across the internet. This data includes information about where emails appear to originate from, even if they have been routed or forwarded through intermediary servers after leaving your primary ESP.
Common causes of unexpected IPs
The most frequent culprit behind unexpected IP addresses on SenderScore, especially for dedicated IPs, is email forwarding. When a recipient forwards an email, it often passes through the forwarding server's infrastructure, which may have its own IP address. This IP can then be associated with your domain's sending activity by reputation monitoring services.
Another less common but possible reason could be misconfigurations or shadow IT. Sometimes, emails might be sent through an unapproved system, or a legitimate system might be misconfigured to send mail in a way that bypasses your primary dedicated IP. This could happen with internal tools, CRM systems, or even older, forgotten sending platforms.
Although you might primarily use a dedicated IP, some ESPs (Email Service Providers) might, in rare circumstances, route certain low-volume or transactional emails through shared IP pools. While this is less common for marketing or bulk sending from a dedicated IP, it's worth considering if you use a provider that offers both options and has complex routing rules.
Likely causes
Email forwarding: Recipient email services forwarding messages to another address can cause new IPs to appear.
ISP-level routing: Some internet service providers (ISPs) may route emails through various servers.
Minor ESP routing: Some transactional email providers might use varied IP addresses based on their internal load balancing.
Less common, but possible causes
Compromised account: If an email account or system sending on your behalf is compromised, it could send emails from an unauthorized IP. This is less likely with a strong DMARC policy in place.
Misconfigured DNS records: Incorrect SPF or DKIM records can sometimes lead to emails being sent from unintended IPs, though DMARC helps catch this.
Leveraging DMARC for investigation
When an unexpected IP appears, your best tool for investigation is DMARC (Domain-based Message Authentication, Reporting & Conformance). DMARC allows you to receive aggregate reports (RUA) that detail all email sending activity for your domain, including the IP addresses involved and their authentication results (SPF and DKIM).
These reports provide crucial visibility. If an email is forwarded and SPF breaks alignment, DMARC will report this. For instance, if a user on Hotmail forwards an email to a Yahoo address, Yahoo's Mail Transfer Agent (MTA) might generate a DMARC report indicating an SPF failure, giving you insight into the forwarding path.
Implementing a DMARC policy, even starting with p=none for auditing, provides an invaluable audit trail of all email streams originating from your domain. This can confirm if the unexpected IP is genuinely sending on your behalf or is merely a byproduct of legitimate mail flow, such as forwarding. You can also explore how to safely transition your DMARC policy over time to enforce stricter policies like quarantine or reject.
Even if an unexpected IP is a result of forwarding, it still impacts your overall domain reputation indirectly. Maintaining a robust email program is key. This includes proper email authentication (SPF, DKIM, DMARC), consistent sending volume, and diligent list hygiene to avoid spam traps and reduce bounce rates. Remember that a new dedicated IP needs to be warmed up gradually to build a positive reputation with ISPs.
Regularly monitoring your IP reputation is also critical. Beyond SenderScore, keep an eye on Google Postmaster Tools and other reputation dashboards. Check for any blocklist (or blacklist) listings associated with your IPs or domain. A sudden appearance on a blocklist can significantly impact deliverability. Understanding when to use dedicated versus shared IPs can also help you optimize your sending strategy for different email volumes and frequencies, further safeguarding your reputation.
Best practices for maintaining IP reputation
Implement DMARC: Use a DMARC record generator to get visibility into all sending sources.
Monitor reports: Regularly review DMARC reports for unexpected IP addresses and authentication failures.
Maintain list hygiene: Clean your email lists to avoid spam traps and inactive addresses.
Conclusion
Discovering an unexpected IP on SenderScore when using dedicated IPs is a common occurrence that typically stems from email forwarding. While it can be disconcerting, implementing DMARC provides the visibility needed to identify and understand these additional sending sources. This allows you to differentiate between legitimate routing and actual unauthorized activity.
By proactively monitoring your email streams and ensuring proper authentication, you can maintain a strong sender reputation and ensure your emails consistently reach the inbox, safeguarding your overall email deliverability. Consider reviewing your email deliverability strategy to address any broader issues.
Views from the trenches
Best practices
Always set up DMARC records to audit all email streams, even if you start with a 'p=none' policy.
Regularly review your DMARC aggregate reports to identify any unexpected sending IPs or authentication failures.
Maintain consistent sending volumes on dedicated IPs to avoid perception of erratic behavior by ISPs.
Clean your email lists often to prevent hitting spam traps and reduce bounce rates, which negatively impact reputation.
Common pitfalls
Misinterpreting unexpected IPs as malicious spoofing without first checking for legitimate forwarding.
Neglecting DMARC reporting, which hides crucial insights into your email ecosystem.
Not warming up a new dedicated IP address gradually, leading to immediate deliverability issues.
Ignoring PTR records or reverse DNS lookups, which are fundamental for IP reputation checks.
Expert tips
Use a reverse DNS lookup tool to identify the owner of any unfamiliar IP address found on SenderScore.
If the unexpected IP belongs to a major ISP, it is highly likely due to legitimate email forwarding.
Gradually increase your DMARC policy from 'p=none' to 'quarantine' or 'reject' using the 'pct' tag.
Always cross-reference SenderScore data with your DMARC reports for a comprehensive view of email origins.
Marketer view
A marketer from Email Geeks says that seeing unexpected IPs on SenderScore happens frequently and is usually not a significant issue.
2018-04-20 - Email Geeks
Marketer view
A marketer from Email Geeks says that DMARC is essential for checking sending patterns and ensuring no one is spoofing the domain from those IPs.
ReturnPath) collect data from various sources across the internet. This data includes information about where emails appear to originate from, even if they have been routed or forwarded through intermediary servers after leaving your primary ESP.
Hotmail forwards an email to a 
Yahoo address, Yahoo's Mail Transfer Agent (MTA) might generate a DMARC report indicating an SPF failure, giving you insight into the forwarding path.
