Oh wow thanks! :) One program syncs my home Mastodon timeline, with all replies, to a Maildir. Dovecot serves that over IMAP. Sending involves a custom SMTP server which reads the mail message and creates a post from it.
For Mastodon it was all about converting statuses (toots? Posts?) into RFC 5322 messages. Using the status’ ID as Message-Id in the message header is handy. Mail clients do the heavy lifting of rendering threads thankfully!