The chat.vim.wtf chatmail relay was impacted by outbound SMTP rate-limiting imposed by the hosting provider, resulting in significant delays in message delivery. Once I realized the issue (and had the free time to do something about it), I migrated the service to another provider which doesn't throttle outbound email delivery. The new server will cost me about an extra dollar per month but should ensure long-term reliability of the service.
I've got a fever and the only prescription is more self-hosting. This time, it's a light-but-seriously-hardened email server optimized for end-to-end encrypted chats.
After a lot of tinkering over the last week or so, this weekend I migrated most of my on-prem container-based workloads to a new 3-node Docker Swarm cluster.