Message-ID email header

What is the "Message-ID" email header?

This email header is described in the RFC 5322. This field provides a unique message identifier that refers to a particular version of a particular message. The mail server that sends the message is responsible for guaranteeing the uniqueness of its Message-ID. It should be machine-readable and not necessarily meaningful for humans.

Why should I use it?

In Google's Bulk Senders Guidelines they mention that:

Messages must have a valid 'Message-ID:' header field

By not complying with this guideline, you increase the chances that your messages will end up in your recipients' Spam folders.

How do I set it up?

In whichever system or software you use to send messages to your subscribers, you need to edit its email header area and include a new field:

Message-ID: %%uniqueID%%

Where %%uniqueID%% is a unique identifier for each email being sent from your system. Note that this applies to every type of email, whether it be an autoresponder, broadcast, or transactional one.

How can I test that it's setup properly?

The "Message-ID" header is one of many things that are validated by the Winning Email domain checkup tool: