The pull request has been updated to mimic net/mail's behavior rather than
trying to workaround user input.

Before:

    >>> email.message_from_string('From: a@malicious.org@important.com',
policy=email.policy.default)['from'].addresses
    (Address(display_name='', username='a', domain='malicious.org'),)

    >>> parseaddr('a@malicious.org@important.com')
    ('', 'a@malicious.org')

After:

    >>> email.message_from_string('From: a@malicious.org@important.com',
policy=email.policy.default)['from'].addresses
    (Address(display_name='', username='', domain=''),)

    >>> parseaddr('a@malicious.org@important.com')
    ('', 'a@')

I like what I saw under the hood, please feel free to hack me for other
tasks in the email stdlib.