@aeva @mhoye The open source rspamd[1] spam filtering system (currently) uses Redis as a purely local cache. I have the impression it was the simplest k/v local store they could use (it certainly didn't need any special configuration when we set the whole system up here). Now that Redis has changed license, future rspamd versions may switch to something else (memcached? I dunno).
1: https://rspamd.com/ and then https://rspamd.com/doc/configuration/redis.html