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PreMailer.Net
C# library that moves your stylesheets to inline style attributes, for maximum compatibility with E-mail clients.
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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.
Up until now I've just been working with CSS which I was able to use in email by finding a tool that converts it to in-line style attributes (https://github.com/milkshakesoftware/PreMailer.Net)
NOTE:
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Hence, a higher number means a more popular project.