If they are new topics, then there shouldn't be too many people being sent emails, as there are no posts to which this new post is a reply to, so the email notification should be triggered for mods / devs (those who want to know about new topics). If the index of the table was corrupt, then that would cause the error, so re-indexing would solve that (hopefully: might have to discard the index and create a new one from scratch).
Secondly, I get the error infrequently (I got it today), but not always when starting a new topic, sometimes when replying, too. And not always when posting a new topic, sometimes it posts normally.