Discord

Discord started as a gaming chat app but has become a general-purpose home for online communities, newsletters included. It handles real-time text and voice chat, organized into channels, and unlike Slack's free tier, it doesn't cap how far back message history goes.

It tends to suit newsletters with a younger or more online-native audience, since Discord's conventions — servers, channels, roles — are already second nature to people who spend time there for other communities, whereas an older or more professional audience might find Slack or Circle more familiar.