How can I group email messages together using Message-ID or VERP?
Michael Ko
Co-founder & CEO, Suped
Published 13 Jun 2025
Updated 15 Aug 2025
9 min read
When analyzing email campaign performance or troubleshooting deliverability issues, the ability to group messages effectively is crucial. It allows for a clearer understanding of how specific sends are performing, whether it is a daily newsletter or a transactional notification series. However, the email ecosystem, governed by various standards and proprietary implementations, makes this task more complex than it might initially seem.
The challenge often lies in deciphering the unique identifiers embedded within email headers. Two primary candidates for grouping messages are the Message-ID and Variable Envelope Return Path (VERP). While both serve as identifiers, their structure, purpose, and reliability for grouping vary significantly across different email service providers (ESPs).
My goal is to shed light on how these identifiers can be used for grouping and what limitations you might encounter. Understanding these nuances is essential for anyone looking to gain deeper insights into their email data, optimize campaign analysis, and resolve deliverability challenges more efficiently.
Understanding the Message-ID
The Message-ID header field is a unique identifier assigned to each email message when it is created. It is designed to be globally unique, much like a serial number for an individual email. The format is typically a string followed by an at-sign and a domain name, resembling an email address. This structure is outlined in RFC 5321, the specification for the Simple Mail Transfer Protocol (SMTP).
While the Message-ID is meant to be unique, its utility for grouping related emails (e.g., all emails from a single campaign) is limited. The primary reason is that there is no universal standard for what information ESPs or mail transfer agents (MTAs) embed within the Message-ID string itself. Many Message-IDs are simply random strings, making it impossible to extract meaningful grouping data. Furthermore, Message-IDs can sometimes be modified or replaced by intermediate mail servers during the transmission process, which means the ID you see in a received email might not be the original one generated by the sender.
Despite these challenges, some ESPs do embed consistent, proprietary patterns within their Message-IDs. For example, a Message-ID might contain a campaign ID, a timestamp, or a unique identifier for the recipient, but this structure is specific to each ESP. To use the Message-ID for grouping, you would first need to identify the sending ESP and then understand their specific Message-ID formatting. This means a universal solution using Message-ID for grouping across multiple senders is impractical. For some senders, if the domain in the Message-ID is on a blocklist, this could also flag deliverability issues unrelated to content.
Variable Envelope Return Path (VERP) is a technique that uses a different Return-Path address for each recipient of an email. This unique return path typically includes encoded information about the recipient and sometimes the specific campaign or message identifier. When an email bounces, the bounce notification is sent back to this specific return path, allowing the sender to accurately identify which recipient caused the bounce. You can find more details about VERP on Wikipedia.
Because VERP is designed to track individual bounces, the string embedded in the Return-Path often contains more structured and decipherable data for grouping. This can include the original recipient's email address (or a hash of it), a unique campaign identifier, or even a customer ID. This makes VERP a more reliable method for grouping messages from the same deployment than Message-ID, especially for large-scale marketing campaigns where bounce attribution is critical. You might find VERP implementation essential for bounce attribution.
While VERP strings are generally more useful for extracting data, their format also varies between ESPs. You would still need to reverse-engineer the encoding scheme of each provider to effectively parse the information. However, the data embedded in VERP is often more consistent within a single ESP compared to the Message-ID. This makes VERP a preferred method when you need to group messages and analyze individual recipient-level outcomes, or troubleshoot return-path issues.
Purpose: A globally unique identifier for each individual email message, as defined by email RFCs.
Location: In the email header, typically formatted as a string followed by a domain name.
Grouping utility
Reliability: Limited for campaign-level grouping due to inconsistent internal structures.
Mutability: Can be altered or replaced by intermediate mail servers, making it less dependable.
VERP basics
Purpose: Enables automatic bounce tracking by using unique return paths for each recipient.
Location: In the email envelope, specifically the Return-Path header.
Grouping utility
Reliability: Higher for campaign and recipient-level grouping due to embedded data.
Consistency: More consistent within a single ESP, often containing campaign-specific parameters.
Practical considerations and challenges
The biggest hurdle in using either Message-ID or VERP for grouping email messages is the lack of a universal standard for their internal structure. While RFCs dictate the overall format, the content within these fields is largely left to the discretion of individual email service providers. This means a Message-ID from ESP A will look very different from one from ESP B, even if both are sending the same campaign content. Similarly, the way VERP encodes campaign or recipient data can vary wildly. This makes it a significant undertaking to build a system that can reliably parse and group messages from diverse sources.
Beyond Message-ID and VERP, some senders leverage custom headers to embed grouping information. These headers, often prefixed with X- (e.g., X-Campaign-ID), allow for complete control over the format and content, making them highly reliable for internal grouping. However, relying solely on custom headers means you can only group emails you send yourself or from ESPs that support and document their custom headers. For some ISPs, like some ISPs, the ability to track replies using custom headers may be impacted if the subject line changes.
Effectively grouping email messages requires either prior knowledge of the ESP's specific encoding schemes or a robust system capable of parsing various formats and extracting relevant data. This can involve significant development effort and ongoing maintenance to adapt to changes in how ESPs structure their headers. It is less about a single resource explaining all nuances and more about a continuous process of data observation and adaptation. For example, grouping Microsoft email provider data for deliverability reporting often requires understanding how their specific headers function.
Understanding header fields
While Message-ID and VERP are prominent, other headers like In-Reply-To and References (for conversations) or List-ID (for mailing lists) can also be useful for grouping. It is crucial to understand which headers your ESP or MTA uses for specific purposes, or to ensure your own system populates them correctly.
Data analysis and impact on deliverability
The primary benefit of grouping email messages, regardless of the method, is enhanced data analysis. By aggregating data points for specific campaigns or email types, you can identify trends, pinpoint issues, and optimize your sending strategy. For instance, if you see a sudden spike in bounces, grouping messages by campaign allows you to quickly isolate which specific send caused the problem. This granularity is essential for maintaining a healthy sender reputation and ensuring your emails reach the inbox.
Effective grouping directly impacts how you handle bounced emails and analyze feedback loops. VERP, in particular, is invaluable for processing bounces, as it allows for automated removal of invalid addresses, thereby improving list hygiene. Without a method to group or attribute bounces, it becomes difficult to understand the true health of your email lists or pinpoint which campaigns are generating high bounce rates. For any bounce messages you receive, this level of detail is a game-changer.
Building a reliable system for email grouping often requires a comprehensive understanding of email headers, parsing techniques, and database management. It is an investment that pays off in cleaner data, more accurate reporting, and ultimately, better email deliverability. While there might not be a single 'magic' resource for every ESP's header nuances, the principles of extracting data from Message-ID and VERP provide a strong foundation for any email program seeking advanced analytical capabilities.
Method
Primary Use
Reliability for Grouping
Ease of Implementation
Message-ID
Unique message identification
Low (inconsistent across ESPs, can be modified)
High (if parsing a single ESP's format)
VERP (Return-Path)
Bounce attribution, recipient tracking
Medium-High (more structured, but still ESP-dependent)
Medium (requires parsing unique string structures)
Custom Headers (e.g., X-Campaign-ID)
Flexible, granular campaign tracking
High (full control over content and format)
Low (requires internal system changes for sending)
Views from the trenches
Best practices
Always prioritize VERP for bounce processing and recipient-level tracking as it provides more reliable data.
If using a specific ESP, investigate their documentation for how campaign or recipient data is encoded in Message-ID or VERP.
Implement custom headers for internal tracking purposes when you have control over the email sending system.
Consistently monitor Message-ID and Return-Path headers to detect unexpected changes from ESPs.
Common pitfalls
Assuming Message-ID is universally structured for campaign grouping across different email service providers.
Overlooking the potential for Message-ID modification by intermediate mail transfer agents (MTAs).
Failing to parse VERP strings correctly, leading to inaccurate bounce attribution and list hygiene.
Relying solely on external tools without understanding the underlying header data.
Expert tips
For campaign-level grouping, VERP strings are often more beneficial than Message-IDs, as they commonly contain useful campaign or recipient information.
Be aware that Message-IDs are typically unique to each email and can be random strings, limiting their utility for broad campaign grouping.
While ESPs might have internal documentation for their Message-ID and VERP structures, this information is not usually publicly available.
Consider that Message-IDs might not always survive the SMTP process untouched, as servers in the path can sometimes modify or replace them.
Marketer view
Marketer from Email Geeks says that while there is no universal standard for Message-IDs, each ESP tends to have its own consistent internal format. Identifying the sender can help unlock the Message-ID's embedded meaning for grouping.
2020-12-18 - Email Geeks
Expert view
Expert from Email Geeks says that Message-IDs are intended to be globally unique and resemble an email address, though many MTAs do not adhere to this. Message-IDs can also be removed or replaced by servers during the SMTP process.
2020-12-18 - Email Geeks
Key takeaways for robust email grouping
Grouping email messages using identifiers like Message-ID or VERP is a powerful way to gain granular insight into your email programs. While Message-ID offers unique message identification, its inconsistent internal structure across ESPs limits its utility for campaign grouping. VERP, conversely, often provides more reliable and structured data for attributing bounces and tracking specific campaign performance, making it a more robust option for analysis. However, both require understanding the specific encoding patterns used by each email service provider.
Ultimately, a combination of leveraging VERP for recipient-level tracking, understanding ESP-specific Message-ID patterns, and potentially implementing custom headers for internal control will provide the most comprehensive grouping capabilities. This proactive approach to data extraction is fundamental for optimizing deliverability, improving campaign performance, and making informed decisions in your email marketing efforts.
