Hi David, the problem is in email._header_value_parser._refold_parse_tree.

Specifically, when the parsetree renders too long, it recursively gets newparts = list(part) (the children). When it does this to a BareQuotedString node, the child nodes are unquoted and unescaped and it just happily serializes these.

I thought I had attached a file that monkey patches the _refold_parse_tree function with a fixed version... let me try again.