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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.
If you have an always on host within the office and accessible to the copiers, even a RPi, there are tons of solutions on Github (ex. https://github.com/decke/smtprelay). A Windows machine I will typically configure hMailServer with MariaDB. For all-cloud, SMTP2Go is really the only solid choice I have found. Others I tried were either more pricey or required a super long key for authentication that copiers did not like.
NOTE:
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Hence, a higher number means a more popular project.