Prefixing lines within an email body that begin "From " with '>' does not work with python3-mailman-hyperkitty 1.2.1-1 (debian) - you may as well delete the remainder of the message.
Simple example: I have a message that contains the line: From 1st April 2019, our turnover has exceeded the threshold for
Unfortunately this breaks hyperkitty_import which sees this as the start of a new message (FYI: check_hk_import reports n+1 messages in the mbox which only contains n messages, and hyperkitty_import fails as there are no subsequent header fields following the "From " such as message-id etc).
Prefixing the line with '>' allows the hyperkitty_import to partially succeed - partially as the line and everything below it is omitted from message in the archive. Presumably because the archiver thinks the remainder is a quote which it omits?
Prefixing with a single whitespace was the best work-around, since the renderer ignores it when viewing the message from the archive.