A Message-ID is a unique identifier assigned to each email to help track and reference the message across mail servers. It is mostly unique-identifier(at)yourdomain.com .The length of a Message-ID depends on its format, but generally, it should not exceed 255 characters, as per RFC 5322.
Basic Message-ID (e.g., <[email protected]>) → ~30–50 characters
Structured Message-ID (with campaign, recipient, client, timestamp, and randomness) → ~70–120 characters
Breakdown of Components
| ID | Components | Purpose |
| 1 | campaignID | Identifies the email campaign |
| 2 | recipientID | Unique ID per recipient (hashed if needed) |
| 3 | clientID | Internal client or sender identifier |
| 4 | timestamp | Ensures uniqueness (nanosecond precision) |
| 5 | random | Additional randomness (6-byte hex) |
| 6 | @domain.com | Matches your sending domain |
What we can include in Structured Message-ID
| Id | Identifies | Approx Length |
| 1 | Campaign-ID | 5–10 chars |
| 2 | Recipien-ID | 8–12 chars |
| 3 | Clien-ID | 5–10 chars |
| 4 | Timestamp | 19 chars (nanosecond precision) |
| 5 | Rando-String | 12 chars (6-byte hex) |
| 6 | Domain-Name | 15–30 chars |
| Total | 70–120 chars |
Benefits of This Approach
Improves Deliverability: Follows Gmail & ESP best practices.
Tracking & Analytics: Easily track messages per campaign, recipient, or client.
Ensures Uniqueness: Timestamp + randomness avoids duplication.
Customizable: Adapt it based on your business needs.
Best Practices
Keep it under 255 characters
Ensure global uniqueness
Use a valid domain
Avoid sensitive data (e.g., email addresses)
Domain Components:-
The Return-Path (Envelope From) is used for bounce handling and is critical for deliverability because it directly impacts SPF authentication. The From address is the visible sender shown to recipients and must align with DKIM for DMARC compliance.
The Message-ID domain identifies the message source but does not directly impact authentication. However, it should ideally match the Return-Path domain to establish trust with receiving mail servers.
Return-Path (Envelope From) and From domain is different
From: Example [email protected]
Return-Path: [email protected]
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=y.eample.com;
Authentication-Results: spf=pass (sender IP is 1.1.1.1)smtp.mailfrom=x.y.eample.com; dkim=pass (signature was verified)header.d=y.eample.com; dmarc=pass action=none header.from=y.eample.com; compauth=pass reason=100
Which Domain Should Be Used in the Message-ID?
In this case, the SPF and DKIM authentication pass successfully, and DMARC aligns with y.eample.com. Because of this, the safest choice for the Message-ID domain is y.eample.com.
Recommended Message-ID Format
To maintain consistency and improve tracking, the Message-ID should be structured using unique identifiers such as a campaign ID, recipient ID, client ID, timestamp, and a random string.
Message-ID:<[email protected]>
Example Generated Message-ID:<[email protected]>
Best Practices for Message-ID Generation Ensure uniqueness by using a combination of timestamp, random string, and tracking identifiers. Use a domain you control, ideally one that aligns with your Return-Path or DKIM domain for consistency. Avoid using free/public domains like Gmail or Yahoo, as this can create authentication issues.
Following these guidelines helps improve email deliverability, authentication alignment, and tracking accuracy.