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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.

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.

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.

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.

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.

18 Dec 2020 - Wikipedia

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