Open-source email client for all platforms: N1

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N1 is a downloadable open-source cross-platform email client (Mac, PC, Linux) that is compatible with all major email providers (Gmail, Yahoo, Icloud, MS Exchange, Hotmail, etc.).

100% free.

My comment: Elegant, easy-to-use and install. N1 is a good alternative to standard email clients especially for those who need to be working often offline. 

More info: https://nylas.com/N1/

Download it here: https://invite.nylas.com/download