Someone mentioned not long ago about how new players are being spammed by bots upon joining, potentially overwhelming them thus contributing to low retention. I could see how this could be the case since, after all, new players attempting to reply will get no response and there's an increasing amount of bots doing this.
You could just set a botting rule, but a mechanical solution could be to disallow unsolicited messages to new players for, say, 10-30 minutes?
You could even go so far as to make an "Prevent Unsolicited Messages" setting for players, similar to how Discord allows you to disallow messages from people who aren't your friend and don't share a server.