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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.
I just tried to run https://github.com/mailcow/mailcow-dockerized on Amazon Lightsail Nano instance $5 with 1GB ram and 40GB hdd, but that's way not enough, simply killed server to the point were I could not ssh to it anymore, had to delete instance.
Last time I found Maddy. I haven't tested it yet, but it looks promising. Docker image is ~27MB only!
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Hence, a higher number means a more popular project.