When configuring DMARC records, we typically encounter a standard set of tags like v, p, rua, and pct. However, occasionally, a less common tag can appear that raises questions, even for experienced users. The !10m tag is one such example, often leading to confusion.
I recently encountered a customer's DMARC record that included the !10m tag, which instantly piqued my interest. Despite its validity, it's not a feature I often see in real-world DMARC implementations. This prompted me to revisit its purpose and share insights into what it means for your email authentication strategy.
In short, the !10m tag is a size limit modifier for DMARC reports. It specifies the maximum size of the report payload that a receiving email server should send to the designated reporting URI. This can be particularly relevant for domains expecting a high volume of reports or having specific limitations on their infrastructure for receiving them.
Let's explore the specifics of this tag, its origins, and the practical implications for email senders and receivers, ensuring clarity around this lesser-known DMARC feature.
Decoding the !size tag in DMARC
The !size modifier is part of the URI scheme for DMARC reporting addresses, specifically within the rua (aggregate reports) and ruf (forensic reports) DMARC tags. Its purpose is to allow the domain owner to indicate to receiving mail servers the maximum acceptable size of DMARC report payloads sent to the specified address. For example, !10m signifies a 10-megabyte limit.
This functionality is defined in RFC 7489, section 6.2, which details the DMARC protocol. The specification allows for size indications using suffixes like 'k' for kilobytes, 'm' for megabytes, and 'g' for gigabytes. If a report generated by the receiver exceeds this specified size, the receiver can either truncate the report to fit the limit or opt not to send it, depending on its implementation. This is particularly useful for managing the load on reporting infrastructures.
Consider this example from the RFC: a URI like mailto:reports@example.com!50m would instruct the receiving server to send aggregate reports to reports@example.com only if the report payload does not exceed 50 megabytes. This allows domain owners to mitigate potential resource strain from excessively large reports, especially forensic reports that can contain more detailed, and thus larger, data sets.
Practical implications and common challenges
Despite its validity within the DMARC specification, the !size tag is rarely observed in active DMARC records. For most domains, even those with significant email volume, aggregate DMARC reports typically remain well within manageable file sizes, making explicit size limitations unnecessary. Forensic reports, while potentially larger, are also often processed by specialized systems that can handle varying file sizes.
One challenge with such obscure tags is that some DMARC validation tools may not correctly interpret them. This can lead to false warnings or errors when checking a DMARC record, causing confusion for administrators. My technical support team has reported instances where validation services fall over or misinterpret records containing the !size modifier, creating unnecessary troubleshooting for domain owners.
However, using a robust DMARC monitoring platform can circumvent these issues. Platforms like Suped are designed to correctly parse and interpret all valid DMARC tags, regardless of their commonality. This ensures that you receive accurate insights and are not misled by validation tools that might struggle with less frequently used features of the DMARC standard.
Best practices for DMARC reporting
Manual DMARC Reporting
Complexity: Requires manual parsing and aggregation of XML reports, which can be time-consuming.
Data overload: Large report files can quickly become cumbersome and difficult to manage without automation.
Limited insights: Extracting actionable trends or specific authentication issues from raw data is challenging.
No alerts: Critical DMARC policy violations or authentication failures might go unnoticed.
Suped DMARC Monitoring
Simplified analysis: Automated parsing, aggregation, and visualization of all DMARC reports.
Manageable data: Efficiently handles large report volumes, ensuring no data is lost or overlooked.
Actionable AI: Provides AI-powered recommendations to quickly resolve issues and optimize your policy.
Real-time alerts: Instant notifications for any DMARC policy violations or authentication failures.
Unified view: Integrates DMARC, SPF, and DKIM monitoring with blocklist insights.
For comprehensive DMARC visibility, collecting both aggregate (rua) and forensic (ruf) reports is essential. While the !size tag can technically be applied to either, it's generally more relevant for forensic reports that could potentially be very large. However, even then, most organizations find that default report sizes are perfectly adequate. Overly restrictive size limits should be avoided unless there's a clear, technical reason to implement them.
When initially deploying DMARC, it is always a best practice to start with a p=none policy. This allows you to collect reports and understand your email ecosystem without impacting email delivery. As you gain confidence in your authentication setup, you can then gradually transition your DMARC policy to quarantine or reject. This phased approach minimizes risks and ensures a smooth DMARC implementation.
For simplified DMARC management, using a dedicated platform like Suped is highly recommended. Suped automates the collection, parsing, and analysis of DMARC reports, including those with less common tags. Our AI-powered recommendations help you understand complex data and take actionable steps to improve your email deliverability and security, regardless of the nuances in your DMARC record. Get started with Suped for free today to streamline your DMARC journey.
Views from the trenches
Best practices
Regularly review your DMARC reports to understand email authentication performance and identify potential issues.
Ensure your DMARC monitoring solution can correctly parse all DMARC tags, including less common ones like !size.
Start with a DMARC policy of p=none to gather data before enforcing stricter policies like quarantine or reject.
Common pitfalls
Setting an unnecessarily low !size limit, which can result in truncated or unsent DMARC reports.
Relying solely on DMARC validation tools that may not properly interpret all DMARC tags, leading to false errors.
Ignoring DMARC reports, which means missing crucial insights into email authentication failures and potential abuse.
Expert tips
If you're using the !size tag, verify that your reporting infrastructure can handle the specified report sizes effectively.
Monitor for patterns in DMARC report sizes to determine if the !size tag is actually beneficial for your specific needs.
Leverage advanced DMARC monitoring tools to gain actionable insights from your reports, regardless of their complexity.
Expert view
Expert from Email Geeks says that the !size flag is valid and specifies the maximum size of the DMARC report message.
2024-04-01 - Email Geeks
Marketer view
Marketer from Email Geeks says they encountered this feature for the first time on a customer's DMARC record and found it surprising.
2024-04-01 - Email Geeks
Final thoughts on DMARC records
While the !10m tag, or more generally the !size modifier, is a valid component of a DMARC record, its practical application is relatively niche. For most organizations, the default behavior of DMARC reporting is sufficient, and the complexity introduced by explicitly setting report size limits is often unnecessary.
The primary goal of DMARC is to protect your domain from impersonation and enhance email deliverability by providing a clear policy for handling unauthenticated messages. Achieving this requires careful configuration of your core DMARC tags and diligent monitoring of your reports. Focusing on these fundamental aspects will yield the most significant benefits for your email security and reputation.
For those seeking to simplify their DMARC journey, Suped offers an unparalleled DMARC monitoring solution. Our platform not only accurately interprets all DMARC tags, no matter how rare, but also provides AI-powered recommendations to guide you through optimization. With a generous free plan and robust features, Suped makes DMARC accessible and effective for everyone, from small businesses to large enterprises and MSPs.
DMARC, SPF, and DKIM monitoring with blocklist insights.
