header_factory
A callable that takes two arguments, name and value, where name is a header field name and value is an unfolded header field value, and returns a string subclass that represents that header. A default header_factory (see headerregistry) is provided that understands some of the RFC 5322 header field types. (Currently address fields and date fields have special treatment, while all other fields are treated as unstructured. This list will be completed before the extension is marked stable.)
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