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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
Just run your own mail server on a Pi at home and get your ISP to set up a PTR record for you, assuming you are on a static IP. Mox is good and outputs all of the instructions for DNS etc.:
https://github.com/mjl-/mox
NOTE:
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Hence, a higher number means a more popular project.