Why is Senderscore showing a subdomain I don't recognize?
Matthew Whittaker
Co-founder & CTO, Suped
Published 13 Jul 2025
Updated 16 Aug 2025
8 min read
It can be unsettling to check your SenderScore and discover a sending subdomain listed that you don't recognize. For instance, you might expect to see something like marketing.yourcompany.com, but instead see bounce.yourcompany.com or another unexpected variant. This situation often leads to confusion about whether your email reputation is at risk or if there's a security issue.
This discrepancy is a common occurrence and usually indicates that the monitoring service (like SenderScore) is picking up on a different aspect of your email's journey than what you typically perceive as your primary sending domain. It's not always a cause for alarm, but it certainly warrants investigation to understand what’s happening behind the scenes of your email delivery.
Understanding why this happens requires a look into the technical components of email sending, particularly the different domains involved in the email authentication process. The good news is that for legitimate senders, an unfamiliar subdomain showing up on a SenderScore report is frequently part of standard email infrastructure operations by your email service provider (ESP).
Understanding email headers and domains
When an email is sent, there are typically two 'From' addresses involved, as defined by different RFCs. The first is the 5322.From address, which is the user-facing 'From' address you see in your inbox, like marketing@yourcompany.com. This is what most people consider the sender. The second, less visible address, is the 5321.From address, also known as the Mail From, Envelope From, or Return-Path address. This address handles bounces and is used for SPF authentication.
SenderScore often reports on the 5321.From domain because it is crucial for how mail servers process email, especially for handling non-delivery reports (bounces). This is why you might see subdomains like bounce.yourcompany.com appear in your SenderScore reports. These are usually automatically configured by your email service provider to manage the technical aspects of email delivery, separating the operational functions from your primary brand identity.
Example of email headerstext
Received: from [xxx.xxx.xxx.xxx] (HELO mail.example.com)
by server.example.com with ESMTPSA id ABCDEF01234567;
Wed, 17 May 2023 10:00:00 -0400 (EDT)
Return-Path: <bounce@bounce.yourcompany.com>
From: "Your Company" <marketing@yourcompany.com>
Subject: Your latest newsletter
To: recipient@example.com
Date: Wed, 17 May 2023 10:00:00 -0400
It's important to differentiate between these two domains. The 5322.From address is what your recipients see, directly impacting their perception of your brand. The 5321.From address, or return-path, is primarily for technical communication between mail servers, and its domain is often a subdomain configured by your ESP. For more on this, you can review details about return-path email headers.
Why unrecognized subdomains appear
Email Service Providers (ESPs) commonly utilize multiple subdomains for different purposes to optimize deliverability and manage email streams effectively. These subdomains are often set up for functions like email tracking (e.g., click.yourcompany.com), bounce handling (e.g., bounce.yourcompany.com), and sometimes even for specific types of email sends, such as transactional or marketing emails.
When SenderScore (or any other reputation monitoring service) reports on a subdomain you don't recognize, it's highly probable that this subdomain is part of your ESP's standard configuration for your sending infrastructure. These subdomains are crucial for maintaining sender reputation. For example, isolating bounce handling to a dedicated subdomain ensures that bounce rates don't negatively impact the reputation of your primary sending domain (or brand domain) if issues arise.
How to inspect email headers
To confirm the role of an unrecognized subdomain, you can inspect the full email headers of a message sent from your domain. In most email clients, this option is typically found under a 'Show original' or 'View source' menu. Look for the Return-Path header, which often corresponds to the subdomain SenderScore is displaying. This can confirm if the subdomain is indeed the bounce domain used by your ESP. For instance, in Gmail, you would click the three dots next to the reply button and select "Show original" to view the headers.
While it might seem strange to have a subdomain you don't directly control appear on a reputation report, this is a standard and often beneficial practice. It helps to compartmentalize different aspects of your email sending, protecting your core brand domain from deliverability issues that might arise from high bounce rates or other technical signals associated with the 5321.From domain.
Impact on sender reputation
SenderScore, a widely used reputation metric by Validity, primarily assesses the reputation of the IP address associated with your sending, but also aggregates data based on sending domains, including those used in the Return-Path header. So, if your bounce.yourcompany.com subdomain has a good sending history with low complaint rates and bounces, its appearance on SenderScore is generally a positive sign, indicating that the mail streams it handles are healthy.
A common misconception is that if the Return-Path domain differs from your main brand domain, it will negatively impact your sender reputation. However, this is generally not the case for well-configured setups. Mailbox providers understand the use of various subdomains for different email functions. What matters most is that all domains involved are properly authenticated with SPF, DKIM, and DMARC, and that they maintain positive sending behaviors.
In fact, many experts advise using a dedicated subdomain for email sending to protect your main domain's reputation. This segregation means that if an issue arises with your email campaigns (e.g., a sudden increase in complaints or spam trap hits), the impact is primarily contained to the sending subdomain rather than affecting your primary website domain. This strategy can actually boost trust and deliverability over time, as recipients begin to recognize and trust your dedicated email subdomain. You can read more about domains and reputation.
Primary brand domain (5322.From)
Visibility: The domain displayed to recipients in the 'From' field of their email client, directly linked to your brand identity.
Purpose: Used for branding and establishing trust with your audience.
Reputation Impact: Directly affected by user engagement (opens, clicks), complaints, and spam trap hits related to the content you send.
Return-path domain (5321.From)
Visibility: Hidden from the end-user, primarily seen by mail servers in email headers.
Purpose: Handles technical communications like bounce messages and is used for SPF authentication.
Reputation Impact: Its reputation can influence deliverability, but a dedicated subdomain helps isolate negative impacts from your primary brand domain.
What to do about it
If you see an unfamiliar subdomain on SenderScore, the first step is typically to verify its purpose. Often, it's the designated Return-Path domain. If your email service provider (ESP) set up your sending infrastructure, this subdomain is usually correctly configured and serves a legitimate function. Contacting your ESP's support team can clarify its role and ensure there are no unintended configurations.
It is generally not advisable to attempt to make the Return-Path domain (5321.From) exactly match your primary From address domain (5322.From). Doing so can break critical email functionalities, especially regarding bounce processing and DMARC alignment. Many ESPs separate these for technical and deliverability reasons, as explained in discussions regarding how subdomains protect domain reputation. If you're encountering deliverability issues, the focus should shift to broader factors rather than solely on this subdomain discrepancy.
Monitoring all sending domains
While SenderScore focuses on IP and Return-Path domains, remember that Google Postmaster Tools also provides insights into domain reputation. If you're using subdomains for different types of mail, it's wise to add them individually to Google Postmaster Tools to get a comprehensive view of their performance and identify any potential issues early. This practice helps monitor each mail stream's health independently. You can find out more in our guide on Google Postmaster Tools Domain Reputation.
If you are indeed experiencing poor deliverability (e.g., emails landing in spam folders or being blocked by filters), it's less likely due to the Return-Path subdomain's presence on SenderScore and more likely related to broader sending practices. Factors such as sending to unengaged or old email lists, high complaint rates, or not adhering to email authentication standards (like SPF, DKIM, and DMARC) are far more common culprits. For issues with deliverability due to potential list quality or engagement, consider a thorough list cleaning or re-engagement campaign, or read more on why emails go to spam.
Views from the trenches
Best practices
Maintain separate subdomains for marketing, transactional, and bounce emails to isolate reputation impact.
Always align your DMARC policy with both SPF and DKIM for all sending domains and subdomains.
Regularly clean your email lists to remove unengaged subscribers and reduce bounce rates.
Common pitfalls
Attempting to force the Return-Path domain to match the From domain, which can break email functionality.
Ignoring reports from SenderScore or Google Postmaster Tools about unfamiliar subdomains without investigation.
Sending to old, unengaged, or purchased email lists, leading to high complaints and blacklists (or blocklists).
Expert tips
Focus on overall deliverability metrics like inbox placement and complaint rates, rather than just a single SenderScore metric.
When encountering issues, first check email headers to understand which domains are being used for authentication and return paths.
Work closely with your Email Service Provider (ESP) to understand their subdomain configurations and best practices.
Expert view
Expert from Email Geeks says that the unfamiliar subdomain is likely the return-path or bounce domain for the mail stream.
2021-04-28 - Email Geeks
Expert view
Expert from Email Geeks says that SenderScore probably picked up the return-path domain, which can be confirmed by viewing the email source code.
2021-04-28 - Email Geeks
Key takeaways
Discovering an unrecognized subdomain on SenderScore can be initially confusing, but it's typically a normal part of how email infrastructure is managed by modern email service providers. These subdomains, often used as Return-Path domains, serve vital technical functions like bounce processing and help maintain the overall health of your email program by isolating different mail streams.
The key takeaway is to understand the distinction between the visible 'From' domain and the behind-the-scenes 'Return-Path' domain. While both contribute to your sender reputation, they do so in different ways. A properly configured Return-Path subdomain is a sign of a robust email setup designed for optimal deliverability.
If you're facing actual deliverability challenges, focus on fundamental email hygiene and authentication. Ensure your email lists are clean and engaged, and that your SPF, DKIM, and DMARC records are correctly implemented for all your sending domains and subdomains. A high SenderScore on an unexpected subdomain is likely a sign of good technical performance, not a problem.
