There are few clear-cut and objective criteria for on-topic vs. off-topic. Putting people under moderation on the base of unclear, non-objective criteria might not the best of ideas if you want to build trust and healthy social interactions. Arbitrariness and biases will probably be lurking in about every corner. Fortunately the on / off -topic is not the most relevant distinction you need to keep the MLs clean. If you need a middle-ground between (dangerously arbitrary, free-speech infringing) moderation and nice but sometimes insufficient abstract rules (such as the Code of Conduct), you can do 2 simple things: 1) Provide a clear path / resolution site for interpersonal conflicts and issues. For example if two people are going two far, you take them apart and iron things out, taming the fire before it spreads over other channels. 1 able person might suffice for doing that for not just the mailing lists but every oS channel. So that instead of punishing, you might end up creating a positive mindset for all parties. 2) Repeat, repeat and repeat again that it's often possible to write emails without referring, explicitely or implicitely, to people; instead you can just refer to things they say, and discuss in a way that minimizes the risk of offending the author. It's not perfect -- not a very heart-warmed way of talking -- but it's often a good temporary solution to keep communication without surrending to the negative emotions bleeding from a wounded ego.