Your mailing application can send custom email headers, which can be a useful way to exchange information using a standardized approach.
You can set up custom email headers to be included in sent mailings by visiting Lists > Custom Email Headers.
Provide an internal name for your custom header (not included in the email header), and the header name and value. You can also use a personalization tag as the value. Choose which lists to use this header, and hit Add.
When a mailing is sent for a list associated with a custom header, the header will appear intermixed among the other headers:
Your application or system can now parse emails for this header, and react accordingly.