email.policy.EmailPolicy.fold_binary()

fold_binary(name, value)

The same as fold() if cte_type is 7bit, except that the returned value is bytes.

If cte_type is 8bit, non-ASCII binary data is converted back into bytes. Headers with binary data are not refolded, regardless of the refold_header setting, since there is no way to know whether the binary data consists of single byte characters or multibyte characters.