Interestingly, mailutils mh (GNU Mailutils 2.99.99, Ubuntu 16.10)
doesn't have a problem with these files, so I'm using that instead of
nmh.

I assume this is relevant:
http://www.nongnu.org/nmh/diff.html
<quote>
The limitation on the length of lines in the file containing your
public sequences (.mh_sequences) has been removed. That should be the
end of the error message ".mh_sequences is poorly formatted". 
</quote>

That's the message I got with nmh, but not with mailutils mh.

Cheers,
Garrett

On Wed, 19 Oct 2016 13:26:33 +0000 "R. David Murray" <report@bugs.python.org> wrote:

> R. David Murray added the comment:
> 
> I meant 'nmh'.  Also, if the problem is that mailbox blows up on an .mh_sequences file with bad records and doesn't provide any way to access the valid records (like nmh mostly manages to do), then that is something that could be fixed, and we can reopen the issue.
> 
> ----------
> versions: +Python 3.6, Python 3.7 -Python 3.5
> 
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> <http://bugs.python.org/issue28473>
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