code: def encode_7or8bit(msg):
      
      orig = msg.get_payload()
      if orig is None:
          # There's no payload.  For backwards compatibility we use 7bit
          msg['Content-Transfer-Encoding'] = '7bit'
          return
      # We play a trick to make this go fast.  If encoding/decode to ASCII
      # succeeds, we know the data must be 7bit, otherwise treat it as 8bit.
      try:
          if isinstance(orig, str):
              orig.encode('ascii')
          else:
              orig.decode('ascii')
      except UnicodeError:
          charset = msg.get_charset()
          output_cset = charset and charset.output_charset
          # iso-2022-* is non-ASCII but encodes to a 7-bit representation
          if output_cset and output_cset.lower().startswith('iso-2022-'):
              msg['Content-Transfer-Encoding'] = '7bit'
          else:
              msg['Content-Transfer-Encoding'] = '8bit'
      else:
          msg['Content-Transfer-Encoding'] = '7bit'
      if not isinstance(orig, str):
          msg.set_payload(orig.decode('ascii', 'surrogateescape'))
docs: Set the Content-Transfer-Encoding header to 7bit or 8bit.