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.

                   to="Albert Johnson <Albert.Johnson@example.com>",
                   subject="An example email",
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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App Engine standard environment for Python