How does Google Postmaster Tools track domain reputation and handle subdomains?
Matthew Whittaker
Co-founder & CTO, Suped
Published 7 May 2025
Updated 17 Aug 2025
7 min read
Google Postmaster Tools (GPT) is a free, indispensable resource for email senders, offering critical insights into how Google perceives your email sending practices. It provides data on various metrics, including spam rate, IP reputation, domain reputation, and delivery errors, all of which are essential for maintaining good email deliverability to Gmail recipients.
Understanding and actively monitoring your domain reputation through Postmaster Tools can make the difference between your emails landing in the inbox or being routed to the spam folder. A strong reputation signals to Google's algorithms that you are a legitimate sender, while a poor one can lead to significant deliverability challenges. This is especially true for marketing and transactional emails where inbox placement is paramount.
A common point of confusion for many senders involves how Google Postmaster Tools handles subdomains. Many organizations use multiple subdomains for different email purposes, such as marketing.example.com for campaigns and trans.example.com for transactional messages. Understanding how GPT tracks the reputation of these subdomains, both individually and in relation to the primary domain, is key to optimizing your overall email strategy.
Understanding Google Postmaster Tools reputation tracking
Google Postmaster Tools primarily tracks domain reputation based on the domain found in the d= (domain) tag of your DKIM (DomainKeys Identified Mail) signature. This means that regardless of the domain in your From: header, GPT will attribute reputation metrics to the domain that cryptographically signed the email. This is a crucial distinction, especially if you use an email service provider (ESP) that signs emails with their own domain, or if you employ multiple subdomains for various sending purposes.
Postmaster Tools categorizes domain and IP reputations into four buckets: High, Medium, Low, and Bad. These ratings reflect Google's trust in your sending practices and directly influence where your emails land in the inbox. A High reputation means your emails are highly likely to reach the inbox, while a Bad reputation almost guarantees delivery to the spam folder or outright rejection. These ratings are influenced by various factors, including spam complaints, bounce rates, and user engagement.
Beyond the d= domain in DKIM, Postmaster Tools also considers SPF (Sender Policy Framework) authentication and DMARC (Domain-based Message Authentication, Reporting, and Conformance) policies. Proper alignment of these authentication protocols is crucial for a healthy domain reputation. If your SPF and DKIM domains align with your From: domain, Google has a clearer picture of your sending authenticity and can assign reputation accurately.
Subdomains and their unique reputation profiles
One of the most frequently asked questions about Google Postmaster Tools is how it handles subdomains. The general rule is that Google evaluates the reputation of each subdomain individually. This means that marketing.yourdomain.com could have a Bad reputation, while trans.yourdomain.com maintains a High one. This granular tracking allows organizations to segregate their email traffic and manage the associated risks.
Using subdomains is a common strategy to protect the reputation of your primary domain. If your marketing emails, sent from marketing.yourdomain.com, start receiving high spam complaints, the negative impact is typically contained to that subdomain. Your transactional emails, sent from trans.yourdomain.com, would likely remain unaffected, ensuring critical communications continue to reach the inbox. We often discuss this separation and its benefits, including how it impacts core domain reputation.
However, there's an important nuance. While Postmaster Tools offers individual data for subdomains, Google's overall compliance status dashboard for a primary domain (e.g., yourdomain.com) uses data from subdomains to determine compliance, even if it only provides the status for the primary domain. This means that a severe, sustained issue on a subdomain can indirectly impact the primary domain's perceived health by Google, even if it doesn't directly alter the High/Medium/Low/Bad rating of the root. This is why it's critical to monitor all your sending entities, including subdomains.
Root domain reputation
Verification: Requires a DNS TXT record verification for the primary domain.
Visibility: Provides an overall compliance status that is influenced by all subdomains. This is an aggregate health indicator.
Purpose: Offers a high-level view of your domain's general standing with Google.
Subdomain reputation
Verification: Often automatic if the root domain is verified, though individual adding is possible.
Visibility: Provides granular data specific to each subdomain's sending patterns and performance.
Purpose: Allows for targeted monitoring and troubleshooting for distinct email streams.
Setting up and monitoring subdomains in Postmaster Tools
To gain insights into your subdomain performance, you must first verify your primary domain in Google Postmaster Tools. Once the root domain is authenticated, you can often add subdomains without additional code verification, provided you're logged into the same Google account that verified the root. This streamlines the monitoring process for organizations with complex email infrastructures.
Even with automatic addition, it's a best practice to actively add and review data for all significant sending subdomains. This ensures you have granular visibility into each email stream. For example, if your promotional emails from promo.yourdomain.com start to experience low inbox placement, you can pinpoint the issue to that specific subdomain and take corrective action without disrupting your transactional emails.
It's also worth noting that if different subdomains use different IP addresses for sending, Google Postmaster Tools will track the reputation for both the domain (via DKIM d= tag) and the specific sending IP. This dual tracking mechanism provides a comprehensive view of your sending health. You can delve into managing subdomain reputation with multiple IPs for more detailed strategies.
Key verification recommendations
Verify all sending domains: Ensure your primary domain and all actively sending subdomains are verified in GPT.
Monitor DKIM d= value: Confirm the domain in your DKIM signature is the one you intend to monitor in GPT.
Check IP reputation: Even if domain reputation looks good, poor IP reputation can affect deliverability.
Impact on email deliverability and best practices
While subdomains offer a layer of protection by isolating reputation, a poor reputation on any subdomain can still indirectly affect your overall email program and brand perception. For example, if your marketing subdomain consistently lands on blocklists (or blacklists), it can lead to higher complaint rates across your entire domain structure, even if other subdomains technically remain clean. Google's systems are sophisticated and can detect patterns of poor sending behavior originating from within your domain family.
The key to sustained email deliverability is to maintain a high reputation across all your sending entities, including all subdomains and their associated IPs. This requires consistent monitoring of your Google Postmaster Tools dashboards, adherence to email best practices, and proactive management of any reputation dips. Ignoring issues on a single subdomain can lead to broader deliverability challenges for your primary domain and your entire email program. Learn more about why emails go to spam.
Views from the trenches
Best practices
Actively monitor all sending domains and subdomains in Google Postmaster Tools for comprehensive insights.
Ensure your DKIM 'd=' tag aligns with the domain you intend to monitor for accurate reputation data.
Segment your email traffic by using different subdomains for different types of emails (e.g., marketing vs. transactional) to isolate reputation risks.
Common pitfalls
Assuming that only monitoring your root domain in GPT is sufficient for all your sending activities.
Not understanding that GPT primarily tracks reputation based on the DKIM signing domain, not necessarily the 'From:' domain.
Ignoring low reputation on a subdomain, thinking it won't affect your primary domain's overall deliverability.
Expert tips
If your ESP signs with their own domain, ask if they can also double-sign with your domain to enable your GPT visibility.
Even if a subdomain is added automatically after verifying the root, periodically confirm its data appears as expected.
A sudden drop in domain reputation often correlates with an increase in spam complaints or a significant change in sending volume.
Expert view
Expert from Email Geeks says Google applies reputation individually to each subdomain, so it is necessary to monitor all of them for a complete picture.
May 27, 2020 - Email Geeks
Expert view
Expert from Email Geeks says Google Postmaster Tools defaults to tracking by the 'd=' value in the DKIM signature, regardless of the sending domain or IP.
May 27, 2020 - Email Geeks
Optimizing your email reputation with Postmaster Tools
Google Postmaster Tools is an invaluable asset for anyone serious about email deliverability to Gmail users. It provides detailed, near real-time insights into your sending reputation, largely driven by the DKIM d= tag. Crucially, it tracks reputation at the subdomain level, allowing for segmented monitoring and risk management across different email streams.
By understanding that each subdomain can have its own reputation, and proactively setting up and monitoring all your sending domains and subdomains within GPT, you empower yourself to identify and resolve deliverability issues efficiently. This comprehensive approach to monitoring is essential for maintaining high inbox placement rates and ensuring your messages consistently reach your audience.
