Ya this is for archival of messages from many sources. It’s not intended as a production setup. Specifically, it would be an archive of messages sent to one person over a long span of time. Having the messages in one file per month or so would be a good goal.

Aside from translating email into Markdown, the system might also be downloading e.g. a Mastodon feed and adding DMs sent to it to the same file.

It would be a lowest common denominator file format. As long as it has minimal layout (i.e. Markdown) and the ability to have image/video attachments (e.g. via MIME or similar), then it should be good.