How can I group email messages together using Message-ID or VERP?
Michael Ko
Co-founder & CEO, Suped
Published 13 Jun 2025
Updated 14 Aug 2025
10 min read
Summary
Grouping email messages is essential for various analytical and operational tasks, from understanding campaign performance to tracking individual message delivery paths. While a globally unique Message-ID exists for each email, its format is not standardized across all Email Service Providers (ESPs). This lack of uniformity can make Message-ID challenging to use for consistent grouping. An alternative is the Variable Envelope Return Path (VERP), which often contains more structured data suitable for tracking and grouping, particularly for bounce processing. However, the exact implementation of both Message-ID and VERP varies significantly between ESPs, requiring a deep understanding of each provider's specific (and often proprietary) header conventions.
Key findings
Lack of Message-ID standardization: The Message-ID field is meant to be globally unique but its internal structure and content (beyond uniqueness) are not consistently defined or implemented across all ESPs.
Message-ID volatility: The Message-ID can sometimes be altered or replaced by mail servers during the SMTP process, making it an unreliable identifier for consistent tracking across the entire delivery chain. This issue could affect your ability to understand deliverability issues.
VERP utility: VERP often contains more structured data about the campaign, recipient, or specific send, making it a more robust option for grouping messages and managing bounces. For instance, it is crucial for proper bounce response management.
ESP consistency: While formats vary between ESPs, they are typically consistent within a single ESP, allowing for programmatic parsing once the specific ESP's patterns are identified. This is particularly relevant if your Message-ID domain is on a blocklist.
Key considerations
Data extraction challenges: Relying on Message-ID or VERP for grouping requires understanding proprietary formats, which can be time-consuming and require ongoing maintenance as ESPs may change their internal structures without notice.
Alternative grouping methods: Consider if other email headers (like X-Mailer, List-ID, or custom headers added by your ESP) might offer more reliable or straightforward ways to group messages.
Focus on deliverability: While Message-ID and VERP are useful for internal tracking, ensure that any modifications or reliance on these elements do not negatively impact core email authentication (like SPF, DKIM, DMARC) or overall email deliverability.
VERP's primary role: Remember that VERP is primarily designed to facilitate the processing of bounced emails by uniquely identifying the recipient that caused the bounce. More information can be found on Wikipedia's page on VERP.
What email marketers say
Email marketers often seek ways to group messages to analyze campaigns, segment audiences, or track specific sends. While the Message-ID seems like a natural candidate, its varied implementation across ESPs (Email Service Providers) poses a significant challenge. Marketers frequently find that what works for one ESP's Message-ID format doesn't apply to another. Instead, VERP (Variable Envelope Return Path) emerges as a more promising, albeit still complex, method for extracting identifiable information to group emails, especially for bounce management. The key is understanding that consistency is often found within a single ESP, rather than universally.
Key opinions
Message-ID inconsistency: Marketers frequently note that there's no industry standard for the internal content of a Message-ID, meaning each ESP rolls their own format.
Internal consistency: While Message-ID is inconsistent between ESPs, it tends to be consistent within a single ESP, allowing for some level of deciphering specific to that provider.
VERP preference: Many marketers find VERP more useful for extracting data to group messages, especially for tasks like tracking deployments or handling bounce information.
Campaign identification: The primary goal for marketers is often to identify and group messages belonging to the same campaign or deployment, which VERP can sometimes facilitate better than Message-ID.
Key considerations
Grouping use case: Determine if grouping is for internal analytics, troubleshooting, or other purposes, as this impacts whether Message-ID or VERP is appropriate. Consider also if audience segmentation could offer another grouping method.
ESP-specific parsing: Be prepared to develop ESP-specific parsing logic if you intend to extract campaign or recipient data from Message-ID or VERP for grouping.
Batching implications: For large sends, if you send in batches, the Message-ID or VERP might help distinguish between different sub-batches within a larger deployment.
Alternative indicators: Consider if unique identifiers embedded in the email body (e.g., tracking pixels, custom URLs) could offer a more reliable and consistent method for grouping across diverse ESPs than header analysis.
Marketer view
Email marketer from Email Geeks notes that there's no standard for what goes into the Message-ID, suggesting each ESP implements it differently.
18 Dec 2020 - Email Geeks
Marketer view
Email marketer from DigitalMarketer Forum suggests that identifying the sending ESP from the IP address is straightforward, and the Message-ID is then used for deeper internal analysis, not ESP identification.
22 Jun 2024 - DigitalMarketer Forum
What the experts say
Email experts weigh in on the complexities of using Message-ID and VERP for grouping email messages. They emphasize that while Message-ID is globally unique by RFC definition, its content is often opaque or randomized by ESPs, making it unsuitable for extracting structured campaign data. Furthermore, Message-IDs can be altered in transit. VERP, however, is generally seen as a more reliable source for recipient- or campaign-specific information, particularly for bounce processing. Experts agree that deciphering these strings is a complex, ESP-specific endeavor, often requiring direct knowledge of an ESP's internal documentation.
Key opinions
Message-ID limitations: Experts stress that while Message-ID must be globally unique according to RFCs, its actual content often consists of opaque or random strings, making it difficult to extract meaningful grouping data. This is relevant to how matching IDs affect deliverability.
VERP superiority: The VERP string is generally considered a more reliable source for capturing useful campaign, recipient, and send-specific information than the Message-ID.
Internal documentation: ESPs typically document their proprietary Message-ID and VERP formats internally, but these are not usually publicly available, making external analysis challenging.
SMTP alteration risk: Experts warn that Message-IDs can be removed or replaced by mail servers during the SMTP process, further diminishing their reliability for consistent grouping.
Key considerations
Complexity of categorization: Categorizing emails based on Message-ID or VERP across multiple ESPs is a significant and challenging project due to the proprietary nature of these identifiers.
Reliance on other headers: When Message-ID and VERP are opaque, experts suggest looking at other header fields that ESPs might use for embedded information.
Dynamic formats: Do not assume that VERP (or Message-ID) formats will remain static over time for any given ESP, requiring continuous monitoring and adaptation. This is part of the challenge in properly setting up SPF and DKIM for email marketing.
VERP for specific data: Leverage VERP for scenarios where recipient-specific tracking or automated bounce processing is critical, as it is designed for these purposes. For detailed information, consult resources like Spam Resource's articles on VERP.
Expert view
Expert from Email Geeks explains that Message-ID is constrained by RFCs to be globally unique and ideally in the form of an email address, although many Mail Transfer Agents (MTAs) do not fully adhere to the format specification.
18 Dec 2020 - Email Geeks
Expert view
Expert from Word to the Wise suggests that Message-ID often consists of random strings, and while some companies embed more information, VERP strings or other header fields are more likely to contain useful campaign or recipient data.
10 Apr 2024 - Word to the Wise
What the documentation says
Official documentation, such as RFCs (Requests for Comments), provides the foundational definitions for email header fields like Message-ID. These documents specify that a Message-ID must be a globally unique identifier for a message. However, the internal format of this identifier beyond its uniqueness is largely left to the implementer (e.g., the ESP or MTA). VERP (Variable Envelope Return Path) is another technique described in various technical documents, primarily focused on improving bounce processing by using a unique return path for each recipient. This allows automated systems to trace bounces back to specific recipients or campaigns without requiring complex parsing of the message body itself.
Key findings
Message-ID definition: RFCs define the Message-ID field as containing a globally unique identifier for an email message. Its primary purpose is to distinguish one message from all others.
VERP functionality: Documentation on VERP confirms its design to use unique return paths for each recipient, enabling automated bounce detection and handling, as detailed by Limilabs.
Internal structure flexibility: While the format of Message-ID should adhere to a specific syntax (like an email address), the internal opaque string before the @ symbol can vary greatly per system generating it.
Bounce management: VERP is a crucial component for efficient bounce management, allowing for precise identification of undeliverable addresses without manual intervention, a key aspect of understanding email deliverability metrics.
Key considerations
RFC compliance vs. implementation: While RFCs lay out the rules, practical implementations by ESPs often introduce unique (and sometimes non-standard) elements into Message-ID and VERP strings.
Parse for specific data: When using Message-ID or VERP for grouping, focus on identifying the specific segments within the string that an ESP consistently uses for campaign or recipient identifiers.
DMARC reporting context: The consistency of these identifiers can also impact how effectively DMARC reports can be analyzed, as they rely on consistent message identification for aggregation.
Evolution of standards: Keep in mind that email standards and their interpretations evolve, requiring ongoing attention to how Message-ID and VERP are used by mail systems.
Technical article
RFC 5322 (IETF Datatracker) specifies that the Message-ID field must contain a single, globally unique message identifier. This identifier is composed of a local part and a domain, ensuring its uniqueness across mail systems.
04 Oct 2008 - IETF Datatracker
Technical article
Wikipedia's page on Variable Envelope Return Path (VERP) clarifies that VERP is a technique employed by mailing list software to automatically detect and remove undeliverable email addresses by utilizing a distinct return path for each message recipient.
