Definition
The Message-ID header is a globally unique string added to every email when it is sent. It looks like <random-string@yourdomain.com>. ISPs use it to thread replies to the original message, deduplicate bounces, and link delivery events across their systems.
Best Practices
Always include a Message-ID in your sends. Most ESPs generate one automatically, but if you are using a custom SMTP setup, ensure your sending software creates a unique ID per message. Duplicate or missing Message-IDs can cause threading issues, incorrect bounce handling, and deliverability problems with some ISPs.
Related Glossary Terms
AOL Mail for Email Marketers
AOL Mail is a legacy email provider with specific deliverability requirements and rendering quirks, now operating as part of the Yahoo+AOL network under shared infrastructure.
BIMI
BIMI (Brand Indicators for Message Identification) is an email standard that allows brands to display their logo next to their emails in supported email clients. It requires DMARC enforcement at quarantine or reject policy.
Bounce Classification
Bounce classification uses SMTP codes (550, 551, 552, 553, 554, 450, 451, 452) and enhanced status codes to categorise permanent and transient delivery failures.
Bounce Rate Threshold
A bounce rate threshold is the percentage of hard or soft bounces at which email platforms and ISPs begin to penalise sender reputation, typically around 2-5%.
Bounce Rate
Email bounce rate is the percentage of emails that were rejected by the receiving server before reaching the recipient. It is a key indicator of list health and data quality.
Email Deliverability
Email deliverability is the ability to land in the recipient's inbox rather than the spam folder. It depends on sender reputation, authentication, list quality, and content practices.