Notice: Over the next few months, we're reorganizing the App Engine documentation site to make it easier to find content and better align with the rest of Google Cloud products. The same content will be available, but the navigation will now match the rest of the Cloud products.

Python 3.10 is now generally available.

google.appengine.api.mail.invalid_headers_reason

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Determines the reason why a header is invalid.

headers Header value to check.

A string that indicates the reason that the headers are invalid if the reason can be determined; otherwise returns None.