What is Message-ID?
Message-ID is a unique identifier assigned to every email, used for threading conversations, tracking delivery, and deduplicating messages across email systems.
Message-ID is a globally unique identifier assigned to each email message, formatted as a string that looks like an email address: <unique-part@domain>. For example: <[email protected]>.
Every email should have exactly one Message-ID, generated by the sending server. It serves multiple purposes: - **Threading**: Email clients use Message-ID with In-Reply-To and References headers to group conversations - **Deduplication**: Servers use Message-ID to recognize duplicate deliveries - **Tracking**: Senders can use Message-ID to correlate delivery events with specific sends - **Debugging**: Message-ID appears in logs, helping trace issues
The uniqueness requirement is critical. If two different emails share a Message-ID, clients may incorrectly thread them or servers may discard one as a duplicate.
Why Message-ID Matters
Message-ID enables proper email threading and tracking. Without unique Message-IDs, replies might not group correctly, duplicate detection fails, and troubleshooting becomes difficult. For transactional email, Message-ID lets you correlate webhook events (delivered, opened, bounced) with specific sends in your application.
How Ark Handles Message-ID
Ark generates unique Message-IDs for every email you send, or you can provide your own via the API. We include the Message-ID in our webhook events and API responses, making it easy to track individual messages through their lifecycle. Our logs use Message-ID for debugging, and you can search by Message-ID in the dashboard.
Frequently Asked Questions
Should I generate my own Message-ID or let the service do it?
Let your email service generate it unless you have specific tracking needs. If you generate your own, ensure global uniqueness. A common pattern: <timestamp.random.yourdomain@your-sending-domain>.
Why do I see multiple emails with the same Message-ID?
Either a bug is generating duplicate IDs (fix immediately), emails are being resent with the same ID (incorrect), or you're seeing the same email multiple times due to forwarding or multiple delivery paths (normal).
Where do I find an email's Message-ID?
In email headers. In Gmail, click the three dots > 'Show original.' In Outlook, open the email > File > Properties. Look for the 'Message-ID' header. Your email service's API response or dashboard should also show it.
How does Message-ID affect email threading?
When you reply to an email, your client adds the original's Message-ID to the In-Reply-To header and appends it to References. Clients use these to build conversation threads. Broken Message-IDs break threading.
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