An email message can include headers. The headers field of an email message accepts a dictionary describing the headers. Each key:value pair of the dictionary corresponds to a header, where the key is the header name, and the value is the value of the header, as a string.

from google.appengine.api import mail

mail.send_mail(sender="[email protected]",
               to="Albert Johnson <[email protected]>",
               subject="An example email",
               body="""
The email references a given email thread id.

The example.com Team
""",
               headers={"References": email_thread_id})

Only certain header names are allowed. For more information, see Sending Mail with Headers .

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