What is c2m_uniqueid

The c2m_uniqueid is a custom, alphanumeric string that you assign to each recipient in your mailing list. It acts as a reliable identifier for individual pieces of mail.

Why is it necessary? When you want to look up tracking data for a specific piece of mail via the API, searching by the recipient's name or address is unreliable:

• Names aren't unique: You might send two mail pieces to the same person (e.g., one to their home, one to their office). A search for "Gina Hansen" wouldn't know which piece you meant.
• Addresses change: USPS automatically standardizes addresses. If you upload a 5-digit ZIP code, USPS might convert it to a ZIP+4 (e.g., 75704 becomes 75704-6644). If the API expects an exact address match, these slight variations will cause your tracking search to fail.

The c2m_uniqueid solves this by giving every address a permanent, unique tag of your choosing (e.g., GinaHansen_CaseID123_Office), entirely bypassing address formatting issues.

How to use c2m_uniqueid

1.Create an Address Mapping:

Before uploading your list, you must create an addressMappingId that includes c2m_uniqueid as one of the fields. (Read the Mail Merge Documentation for details on creating this).

2.Include the ID in your address list payload:

When uploading your mailing list, include the <c2m_uniqueid> tag for every recipient. You can use any combination of strings or numbers, as long as every recipient's ID is unique.

XML

<addressList>

<addressListName>JEN-209-REUSE</addressListName>

<addressMappingId>9385</addressMappingId>

<addresses>

<address>

<Name>Joe Postal</Name>

<Street>500 W Overland Ave</Street>

<City>El Paso</City>

<State>TX</State>

<Zip>79901</Zip>

<c2m_uniqueid>JoePostal_CaseID123_Office</c2m_uniqueid>

</address>

<!-- Add subsequent addresses with their own unique IDs here -->

</addresses>

</addressList>

3.Retrieve tracking data:

Once the job has been mailed, use the unique ID to make a highly specific API call to get tracking information for just that piece of mail:

HTTP

GET /jobs/{jobId}/uniqueid/tracking?trackingType=IMB&trackingDetail=true&uniqueId=JoePostal_CaseID123_Office

Documentation related to this:

Request USPS Tracking with Unique ID

or

Request USPS Electronic Return Receipt with Unique ID