Should both top-level and DKIM subdomains be monitored for email deliverability?
Michael Ko
Co-founder & CEO, Suped
Published 16 Apr 2025
Updated 18 Aug 2025
6 min read
When managing email deliverability, a common question arises regarding which domains require attention. Specifically, if you send emails from your top-level domain but sign them with a DKIM authenticated subdomain, should you be monitoring both, or does one suffice? This is a critical point, as many believe that with tools like Google Postmaster Tools V2, everything rolls up to the top-level domain, making separate monitoring unnecessary. However, this perspective overlooks nuanced aspects of how sender reputation and complaint feedback are handled.
The short answer is yes, you should monitor both. While there's a degree of reputation inheritance, distinct email streams, especially those authenticated with different DKIM domains, can develop their own reputations. Ignoring one could lead to hidden deliverability issues that impact your overall email program.
Understanding domain roles in email authentication
To grasp why both top-level and DKIM subdomains need monitoring, it is essential to understand the different domains involved in email sending. The most visible domain is the From header domain (RFC 5322.From), which is what recipients see. Beneath the surface, the Return-Path domain (RFC 5321.MailFrom) handles bounce messages, and the DKIM signing domain (d= tag) authenticates the message's origin and integrity. Each of these can influence your sender reputation independently.
The reputation of an email sender is not solely tied to a single domain, but rather to a combination of factors. While the DKIM signed domain plays a significant role, particularly in DMARC alignment, other domains and IP addresses also contribute to the overall sender trustworthiness. This multifaceted nature means that issues with one domain, even a subdomain, can have ripple effects that are not immediately apparent if only the top-level domain is monitored.
Using subdomains for different mail streams (e.g., marketing.yourdomain.com, transactional.yourdomain.com) is a widely recommended practice. This strategy allows for isolating sender reputation, so if one stream experiences deliverability issues, it does not immediately jeopardize the others. This is why it is important to understand why you should use subdomains for email marketing.
Why both domains need monitoring
Even when your primary sending is from the top-level domain and DKIM authentication is on a subdomain, monitoring both is crucial. Different mailbox providers (MBPs) may assign reputation based on varying signals, which could include the RFC 5322.From domain, the RFC 5321.MailFrom domain, or the DKIM signing domain. Relying solely on the top-level domain for monitoring might mask problems occurring at the subdomain level, impacting specific email campaigns or transactional flows.
Monitoring the top-level domain only
Limited visibility: May miss specific deliverability issues tied to a particular subdomain.
Reputation blind spots: A subdomain could be on a blacklist (or blocklist) without your top-level domain showing signs of trouble.
Delayed issue detection: By the time issues impact the top-level domain, significant damage may have occurred.
Monitoring both domains
Comprehensive insights: Gain a holistic view of your sender reputation across all sending entities.
Proactive issue resolution: Identify and address subdomain-specific problems before they escalate.
Granular data: Access more precise data points on complaints, spam rates, and bounces for each distinct sending entity.
Complaint feedback and reputation variances often manifest at the subdomain level. A transactional subdomain might have an excellent reputation, while a marketing subdomain could struggle due to engagement issues. If you only monitor the top-level domain, these critical distinctions might be lost, preventing you from accurately diagnosing and resolving deliverability challenges. This granular insight is essential for maintaining strong email deliverability.
DKIM alignment and DMARC's influence
DKIM alignment is fundamental to email authentication. For an email to pass DKIM, the domain in the d= tag of the DKIM signature must align with the RFC 5322.From domain. This alignment can be either strict or relaxed. While some believe that reputation is primarily tied to the DKIM signed domain, it is more complex. Mailbox providers assess reputation based on a blend of signals, including SPF, DKIM, and DMARC alignment, alongside content and user engagement.
The impact of DMARC on subdomains
DMARC (Domain-based Message Authentication, Reporting, and Conformance) is designed to ensure that the domain in the RFC 5322.From address aligns with either the SPF or DKIM authenticated domain. Crucially, a DMARC record published on your top-level domain automatically applies to all its subdomains, including those that do not explicitly have their own DMARC records. This means you typically do not need separate DMARC records for each subdomain. However, you can specify a 'sp' tag in your DMARC record to define a separate policy for subdomains, offering more granular control.
Even with DMARC's overarching policy, the distinct reputation of a DKIM subdomain can still significantly influence deliverability. If a DKIM subdomain is used for high-volume marketing sends that accumulate complaints, its individual reputation could suffer, potentially leading to increased spam classifications for emails signed by it, even if your top-level domain appears healthy. Therefore, continuous monitoring of DMARC reports for both top-level and DKIM subdomains is vital for identifying these subtle reputation shifts. Google and Yahoo now mandate SPF, DKIM, and DMARC for bulk senders. For more information, check this Google Workspace Admin Help article.
Implementing a comprehensive monitoring strategy
To ensure robust email deliverability, your monitoring strategy should encompass all relevant domains and subdomains. This includes regularly checking DMARC reports, which provide aggregate and forensic data on email authentication results across your sending infrastructure. Pay close attention to any authentication failures, particularly those related to DKIM, and investigate the source of these issues immediately.
Specific complaint rates, spam trap hits, reputation metrics from Google Postmaster Tools.
Identifies issues localized to specific sending streams.
Blocklists (Blacklists)
Checks for both domain and IP blocklisting across common public and private lists.
Early warning of critical reputation damage.
Regularly checking Google Postmaster Tools for both your top-level domain and any significant DKIM subdomains is paramount. While some data might consolidate, unique insights into spam rates and complaint feedback loops are often available per verified domain, including subdomains. This allows for a more granular understanding of your sending performance and helps pinpoint the exact source of any deliverability issues, whether they stem from a specific sending platform or email campaign.
Views from the trenches
Best practices
Always verify both your top-level and DKIM subdomains in Google Postmaster Tools for comprehensive data.
Segment your email streams by using distinct subdomains for marketing, transactional, and other email types.
Monitor DMARC reports regularly for all your domains to catch authentication failures early.
Common pitfalls
Assuming top-level domain reputation covers all subdomains, leading to hidden deliverability issues.
Not configuring DMARC, SPF, and DKIM properly for all sending domains and subdomains.
Ignoring complaint feedback loops specific to subdomains, resulting in persistent reputation problems.
Expert tips
Use a different subdomain for every platform you send from to make investigations easier if an issue arises.
Prioritize DMARC monitoring across all domains and subdomains to maintain email security and deliverability.
Regularly check blocklists (or blacklists) for all your sending IPs and domains, including subdomains.
Expert view
Expert from Email Geeks says that it is important to monitor subdomains because they can have different reputations from the parent domain. For example, a client's parent domain might show a medium reputation, while their transactional domain is high and their marketing domain is low.
2024-03-15 - Email Geeks
Expert view
Expert from Email Geeks says that reputation is tied to everything in the email sending process. Using different subdomains for each sending platform or distinct mail stream simplifies investigations when deliverability issues occur.
2024-03-15 - Email Geeks
The importance of comprehensive domain oversight
In the complex landscape of email deliverability, a holistic approach to domain monitoring is not just a recommendation, but a necessity. While DMARC policies offer a protective umbrella over your entire domain space, the individual reputation of your top-level domain and any DKIM subdomains can vary significantly. Ignoring one for the other creates blind spots that could lead to unexpected deliverability issues, affecting your inbox placement and overall email program performance.
Therefore, even if you are sending from your top-level domain and signing with a DKIM subdomain, it is always best practice to verify and actively monitor both in tools like Google Postmaster Tools. This dual approach provides the granular insights needed to proactively identify, diagnose, and resolve potential deliverability challenges, ensuring that your emails consistently reach their intended recipients. Comprehensive email authentication is key.
Google Postmaster Tools.
