Thanks for helping to triage this, Pedro. I think there is a typo in your example: ``email.message_from_binary_file(fp)`` needs to be passed to ``bs.flatten()``.

With the following script I'm also unable to reproduce the issue in Python 3.4+:

    import email
    import email.generator
    import sys

    with open('flatten-exception.mail', 'rb') as f:
        msg = email.message_from_binary_file(f)
        gen = email.generator.BytesGenerator(sys.stdout.buffer)
        gen.flatten(msg)

Hans-Peter, could you share a reproducer with us? Thanks!