@girish thanks for the reply! That's the thing, I'm not a 100% sure yet. It's a custom app that sends out 100+ emails (and growing) every few weeks/months and it hit twice the Connection denied. recipient rate limit exceeded and once Connection denied. connection rate limit exceeded. It wasn't great, because it wasn't clear which mails went through and which didn't (other than manually combing through the logs).
My guess would have been [rate_conn] and [rate_rcpt_host] for starters, but the quick fix (other than building a queue/delay/throttle which adds a lot of complexity) was to use Postmark directly instead of Cloudrons mail-server, so for this specific project this is "solved", but would still be a nice to have feature to adjust the limits in the settings.