[issue27321] Email parser creates a message object that can't be flattened

[Barry A. Warsaw]

Barry A. Warsaw added the comment:

While I was reviewing https://gitlab.com/mailman/mailman/merge_requests/197/diffs I noticed the KeyError and it made me thing "hmm, I wonder if this should be turned into one of the email package errors"?

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27321>
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