Summary
Grouping email messages using Message-ID or VERP involves a multifaceted approach, leveraging email headers to identify relationships. While the Message-ID is intended as a unique identifier, its reliability varies due to ESP-specific implementations and potential alterations during SMTP transmission. In-Reply-To and References headers facilitate email threading, but their consistent implementation across systems is not guaranteed. VERP offers a means to track bounces and group related messages, but its effectiveness can be platform-dependent and may require specialized knowledge for deciphering opaque strings. For greater control, embedding custom campaign identifiers in Message-ID or custom headers can be a more dependable method. Ultimately, successful grouping often necessitates a combination of these techniques, considering the limitations and nuances of each.
Key findings
- Message-ID Variability: Message-ID's uniqueness and reliability are not consistent across all ESPs, and it may be modified during SMTP transmission.
- Threading Header Inconsistencies: In-Reply-To and References headers are intended for threading, but their implementation varies across email clients, impacting reliability.
- VERP Benefits and Drawbacks: VERP facilitates bounce tracking and message grouping, but its effectiveness depends on the platform, and the strings can be opaque.
- Custom Header Reliability: Embedding custom campaign identifiers in Message-ID or custom headers offers a more reliable method for grouping, especially when control over the sending process exists.
Key considerations
- ESP Implementation: The chosen grouping method's success hinges on the specific ESP's header implementation and Message-ID generation.
- Header Manipulation Risks: Standard headers are susceptible to manipulation, potentially compromising the accuracy of email threading and grouping.
- VERP Complexity: Deciphering VERP strings can be complex, potentially necessitating specialized knowledge and tools.
- Control Requirement: Implementing custom headers necessitates control over the email sending process.
- Recipient-Specific MessageIDs: Some ESPs generate recipient-specific message IDs, which are unsuitable for campaign-level grouping.
What email marketers say
10 marketer opinions
Grouping email messages using Message-ID or VERP involves analyzing email headers to identify relationships between messages, but there's no single foolproof method. While Message-IDs are intended to be unique, their predictability varies across ESPs. The 'References' and 'In-Reply-To' headers are designed for threading but aren't consistently implemented. VERP is useful for bounce tracking and can aid in identifying related messages. Custom headers offer more control for grouping messages by campaigns, supplementing standard headers.
Key opinions
- Message-ID Uniqueness: Message-IDs should be globally unique, but predictability varies across ESPs.
- Threading Headers: 'References' and 'In-Reply-To' headers are designed for threading but aren't consistently implemented by email clients.
- VERP for Bounce Tracking: VERP is useful for bounce tracking and can aid in identifying related messages.
- Custom Headers: Using custom headers with campaign identifiers provides more reliable grouping control.
Key considerations
- ESP Variability: The reliability of Message-ID-based grouping depends on the ESP's implementation.
- Header Manipulation: Standard headers can be manipulated or missing, affecting threading accuracy.
- VERP Complexity: Implementing and parsing VERP requires careful handling of return path addresses.
- Custom Header Implementation: Requires control over the sending process to implement custom headers effectively.
- Recipient Specific Message IDs: Some message ID's are designed to be recipient specific and are therefore not campaign specific
Marketer view
Email marketer from Reddit suggests using a combination of `Message-ID`, `In-Reply-To`, and `References` headers to reconstruct email threads. However, notes that these headers can be manipulated or missing, so it's not a foolproof method.
25 Jul 2024 - Reddit
Marketer view
Email marketer from Litmus explains Email clients use a combination of the subject line, and References/In-Reply-To headers. The References/In-Reply-To headers are the most accurate way to thread email messages.
5 Oct 2021 - Litmus
What the experts say
4 expert opinions
Grouping email messages using Message-ID or VERP can be achieved by analyzing email headers. Message-IDs, while meant to be globally unique, may not always be reliable due to varying implementations and potential removal during the SMTP process. VERP strings are often recommended as they contain more campaign and recipient information, especially when using platforms like Salesforce. In-Reply-To and References headers also contribute to email threading. Custom headers can provide internal tracking capabilities.
Key opinions
- Message-ID Limitations: Message-ID uniqueness is not guaranteed, and can be removed during SMTP process.
- VERP Recommendation: VERP strings are more likely to contain useful campaign and recipient information, making them suitable for grouping.
- Threading Headers Importance: In-Reply-To and References headers are crucial for email threading.
- Custom Headers for Internal Tracking: Custom headers and campaign IDs can help for internal tracking where standard methods are insufficient.
Key considerations
- VERP Opacity: VERP strings can be opaque, requiring specialized knowledge to decipher.
- Platform Dependency: The effectiveness of VERP may depend on the email platform used (e.g., Salesforce).
- Header Implementation Variability: Not all systems implement threading headers correctly, impacting reliability.
- MessageID RFC Compliance: Many MTAs violate the email address constraint for MessageID.
Expert view
Expert from Email Geeks mentions the data is probably in the VERP string, but it may be more or less opaque.
28 Jul 2022 - Email Geeks
Expert view
Expert from Word to the Wise explains that Message-ID should be unique for each email. For threading, the In-Reply-To and References headers are important. Not all systems correctly implement these, impacting threading reliability. Custom headers and campaign IDs can help for internal tracking.
23 Aug 2021 - Word to the Wise
What the documentation says
5 technical articles
Grouping email messages can be achieved using both Message-ID and VERP. The Message-ID is a unique, immutable identifier for each email. Email threads can be correlated using the `In-Reply-To` and `References` headers, with `In-Reply-To` pointing to the parent Message-ID and `References` containing a chain of Message-IDs. VERP modifies the Return-Path, embedding recipient-specific information to track bounces and group related events. By analyzing the VERP, you can identify recipients and group messages.
Key findings
- Message-ID Uniqueness: Message-ID is a globally unique and immutable identifier.
- Threading with Headers: `In-Reply-To` and `References` headers are used to correlate messages in a thread.
- VERP for Bounce Tracking: VERP embeds recipient-specific information in the return path to track bounces.
- VERP Grouping: Analyzing VERP allows for identifying recipients and grouping messages related to bounce events.
Key considerations
- Message-ID in isolation: Grouping based solely on Message-ID may not capture threaded conversations.
- VERP Implementation: Implementing VERP requires modifying the Return-Path address.
- Compliance: Adhering to RFC standards for Message-ID formatting ensures interoperability.
- Chain analysis: Correctly analyzing VERP return path data and References header information is required
Technical article
Documentation from LimiLabs explains VERP (Variable Envelope Return Path) which modifies the Return-Path address for each recipient. This allows senders to identify the exact recipient that caused a bounce, aiding in list management and grouping messages related to bounce events.
7 Jan 2022 - LimiLabs
Technical article
Documentation from RFC Editor explains that the Message-ID field contains a single unique message identifier. It is globally unique and immutable over the lifetime of the message. The format is addr-spec (e.g., <local-part@domain>).
14 Mar 2023 - RFC Editor
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