Do matching IDs improve email deliverability to Microsoft accounts?
Michael Ko
Co-founder & CEO, Suped
Published 5 Aug 2025
Updated 19 Aug 2025
9 min read
There's a persistent question in the email deliverability world about whether including matching IDs in emails can improve their chances of reaching Microsoft accounts, like Outlook.com or Hotmail. My experience and current industry understanding suggest that such IDs, often found in X-headers like X-RP-Campaign-ID, are primarily for internal tracking by specific email monitoring or marketing platforms. They typically do not directly influence how Microsoft’s spam filters or mail systems decide to deliver your email.
The idea behind them might stem from a general belief that more unique identifiers could signal legitimacy, but in reality, mail service providers (MSPs) like Microsoft focus on a very different set of signals to determine an email's trustworthiness. For example, recent changes by Microsoft and Yahoo align on DMARC being essential, and this focus on authentication is far more impactful than any custom matching ID.
The true purpose of matching IDs
Matching IDs, often appearing as X-Headers in an email’s technical header, are non-standard fields that providers can use for various purposes. In the context of email deliverability platforms, these might be used to correlate specific email campaigns with performance data. For instance, a platform might insert a unique ID into each email to help its users track how individual campaigns perform in terms of inbox placement, open rates, or click-through rates within their own reporting dashboards.
However, these identifiers are typically opaque to receiving mail servers, including those operated by Microsoft. They don't provide any verifiable information about the sender's legitimacy or adherence to email sending best practices. Instead, Microsoft and other major MSPs rely on well-established email authentication protocols and sender reputation metrics to evaluate incoming mail.
While these X-headers can be useful for internal analytics or for debugging by the sending platform, they are not a factor that Microsoft considers when deciding whether to deliver an email to the inbox or the spam folder. Misunderstanding their purpose can lead senders to focus on irrelevant technical details instead of the core elements that truly impact deliverability.
What microsoft truly prioritizes
Microsoft, like Google and Yahoo, places a high emphasis on several key factors to ensure email deliverability. These factors are continuously refined to combat spam and phishing attempts. Ignoring them in favor of non-standard identifiers will likely result in poor inbox placement.
Authentication standards are paramount. This includes proper configuration and adherence to Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting, and Conformance (DMARC). Microsoft uses these to verify that the sender is legitimate and authorized to send emails from the domain they claim to represent. Without strong authentication, emails are highly likely to be flagged as suspicious, even if your content is perfect.
Sender reputation is another critical component. Microsoft maintains its own reputation systems, including the Exchange Online Protection (EOP) and SmartScreen filters, which assign SCL (Spam Confidence Level) and BCL (Bulk Complaint Level) scores. These scores are influenced by factors like spam complaint rates, bounce rates, user engagement, and whether your sending IP or domain appears on any public or private blocklists (or blacklists). Maintaining a good sending history and high engagement is crucial. To improve email deliverability to Microsoft, you need to focus on these fundamental areas.
Finally, content quality and adherence to email sending guidelines are vital. This includes ensuring your emails are not perceived as spammy, avoiding suspicious links, maintaining a clean mailing list, and honoring unsubscribe requests promptly. For more detail on these requirements, you can refer to Microsoft's own guidelines on authenticating outbound email.
Misconceptions about matching IDs
Deliverability Impact: Many believe these IDs enhance inbox placement. In reality, they do not.
Microsoft Recognition: The belief is that Microsoft's filters recognize and trust these specific headers. They primarily rely on standard authentication.
Universal Benefit: The assumption that a specific ID format is universally beneficial across all mailbox providers, which isn't true.
Sender Reputation: A consistent positive sending history, low complaint rates, and avoiding blacklists are key.
Content Quality: Clean, relevant content without spam trigger words or suspicious links helps improve deliverability.
The importance of email authentication (SPF, DKIM, DMARC)
The foundation of good email deliverability to Microsoft accounts, and indeed to any major mail provider, lies in robust email authentication. SPF, DKIM, and DMARC work together to verify that your emails are truly coming from your domain and haven't been tampered with in transit. Without these, your emails are at a significant disadvantage.
SPF (Sender Policy Framework) allows you to specify which IP addresses are authorized to send email on behalf of your domain. DKIM (DomainKeys Identified Mail) provides a way to cryptographically sign your emails, allowing receiving servers to verify that the email hasn't been altered and truly originated from your domain. DMARC (Domain-based Message Authentication, Reporting, and Conformance) builds on SPF and DKIM, giving domain owners the ability to tell receiving mail servers how to handle emails that fail authentication checks, and providing reports on authentication failures. These protocols are essential. For example, understanding how RFC 5322 aligns with DMARC is key to effective implementation.
Microsoft explicitly states that properly authenticated email is a strong signal for good deliverability. They view emails that pass SPF, DKIM, and DMARC checks as more trustworthy. In fact, Microsoft's new sender requirements emphasize DMARC as a critical component, meaning that if you don't have these set up correctly, your emails will likely face significant deliverability challenges, including landing in the spam folder or being rejected entirely. Here's an example of a simple DMARC record that you might implement:
This record sets a policy of p=none, meaning that emails failing DMARC authentication will not be blocked, but you'll receive reports to help you understand your email flow. This is a common starting point for DMARC implementation, allowing you to monitor before enforcing stricter policies.
Sender reputation and content factors
Beyond authentication, your sender reputation plays a massive role in how Microsoft processes your emails. This reputation is built over time based on various signals related to your sending behavior and the quality of your emails. A good reputation means your emails are more likely to bypass spam filters, while a poor one can lead to throttling, spam folder placement, or outright rejection.
Key factors contributing to your sender reputation include low spam complaint rates, minimal bounce rates, and consistent sending volumes. Sudden spikes in volume or high complaint rates can trigger alarms at Microsoft. It's also vital to ensure your sending IP addresses and domains are not listed on any major blocklists or blacklists, as these are often used by MSPs to filter out known sources of spam. If you're concerned about your domain reputation, there are tools and practices to help understand and improve it.
Content quality is another area where many senders fall short. Microsoft's SmartScreen filter analyzes email content for spammy characteristics. This includes excessive capitalization, too many images, broken links, or misleading subject lines. Ensure your email content is relevant, well-formatted, and provides clear value to the recipient. A clean and responsive HTML structure, combined with concise and engaging text, contributes positively to your deliverability.
Adhering to Microsoft's guidelines also means maintaining a healthy mailing list. Regularly remove inactive or invalid email addresses to reduce hard bounces and the risk of hitting spam traps. A well-managed list demonstrates responsible sending practices, which Microsoft values highly. Overall, a holistic approach to email deliverability, encompassing both technical setup and sender behavior, is what truly makes a difference.
Views from the trenches
Best practices
Ensure SPF, DKIM, and DMARC are correctly set up and aligned, as these are Microsoft's primary authentication signals.
Consistently monitor your sender reputation and address any issues like high bounce rates or spam complaints promptly.
Maintain a clean and engaged email list, regularly removing inactive or invalid addresses to avoid spam traps.
Craft clear, valuable, and non-spammy email content to improve recipient engagement and reduce spam filter triggers.
Common pitfalls
Relying on non-standard X-headers or internal tracking IDs, believing they boost deliverability to Microsoft accounts.
Neglecting to implement or correctly configure email authentication protocols like DMARC, leading to emails being flagged as suspicious.
Sending inconsistent volumes or frequencies, which can negatively impact your sender reputation and trigger throttling.
Ignoring high spam complaint rates or frequent bounces, which are strong negative signals to Microsoft's filters.
Expert tips
Focus on the core deliverability pillars: strong authentication, positive sender reputation, and quality content.
Use tools like Microsoft Postmaster Tools to get direct feedback on your email performance with Microsoft.
Prioritize list hygiene to keep your engagement rates high and minimize potential negative signals.
Understand that custom IDs are for internal tracking, not for improving deliverability with mailbox providers.
Expert view
Expert from Email Geeks says matching IDs are unique to particular platforms like Return Path and are primarily used for identifying campaigns in Inbox Monitor reports. They do not offer any benefit to deliverability.
2018-11-15 - Email Geeks
Expert view
Expert from Email Geeks says a random X-header, specifically used for Inbox Monitor reporting, is unlikely to be given any weight by mail service providers (MSPs) for deliverability purposes.
2018-11-15 - Email Geeks
Key takeaways for Microsoft deliverability
In summary, while specific matching IDs might be valuable for internal campaign tracking or analytics within certain email platforms, they do not directly improve email deliverability to Microsoft accounts. Microsoft's robust filtering systems prioritize legitimate email authentication, a strong sender reputation, and high-quality content. Focusing on these core elements is the most effective way to ensure your emails reliably reach the inbox.
As the email landscape continues to evolve with stricter sender requirements, especially from major providers like Microsoft, it’s more crucial than ever to adhere to established best practices. Investing time and resources into proper SPF, DKIM, and DMARC setup, maintaining a healthy sending reputation, and consistently delivering valuable content will yield far greater improvements in your deliverability than relying on proprietary matching IDs.
Microsoft accounts, like Outlook.com or Hotmail. My experience and current industry understanding suggest that such IDs, often found in X-headers like X-RP-Campaign-ID, are primarily for internal tracking by specific email monitoring or marketing platforms. They typically do not directly influence how Microsoft’s spam filters or mail systems decide to deliver your email.
Google and Yahoo, places a high emphasis on several key factors to ensure email deliverability. These factors are continuously refined to combat spam and phishing attempts. Ignoring them in favor of non-standard identifiers will likely result in poor inbox placement.
