You might be interested in Topicbox, which we (FastMail) have built for precisely this reason. We use Slack for our real-time comms, but we still used mailing

You might be interested in Topicbox, which we (FastMail) have built for precisely this reason. We use Slack for our real-time comms, but we still used mailing lists for more thoughtful discussion and documenting decisions, so we made mailing lists unsucky: https://www.topicbox.com/ https://www.topicbox.com/ It's still in soft-launch at the moment while we finish off a couple of key features that we identified in beta.