A Matching ID is a unique identifier assigned to each mailing that Return Path uses to accurately track and report on customers' email.

A Matching ID should be placed in either the header of a message (recommended) or the body of the message, but not both, as only one ID is required to track and report on a customer’s email.

Many email service providers (ESPs) or mail transfer agents (MTAs) generate a unique identifier for you in the header. Your ESP or MTA can tell you if this is the case.

• If it is, you almost never need to set up a separate Matching ID.
• If it is not, check with your ESP or MTA to see if you can add a Matching ID to your email header. If you can’t, you will need to insert one in the message body. (See How to set up a Matching ID.)

In either case, review your Matching ID setup whenever you migrate to a new ESP or MTA. This helps prevent your messages from being tracked using old IDs.

How Return Path chooses a Matching ID

Sometimes a Matching ID is placed in both the header and body of a message. For example:

• Your ESP can’t accommodate a Matching ID in the email header, so you include the default Matching ID in the message body.
• You migrate to an ESP that generates a unique identifier for you in the header, but your templates still include the default ID in the body.
• Now you have an old, default Matching ID in the body and a new, ESP-generated ID in the header.

Return Path uses logic to determine which single Matching ID to retrieve and record. A series of regular expression searches are run against the message, first looking for the default Matching ID in the header, then in the body. If no ID is found, more searches look for an ESP- or MTA-generated ID or a custom ID in the header and the body.

In the example above, the default Matching ID in the body will be found first, even though there is an ID in the email header.

The same ID location is used to match CoreSeeds and SmartSeeds campaign data.