Can you tell me if there is planned development of an OSX client?

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There is, and it is currently in beta testing, though I understand it is aimed at macOS rather than OS X.

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Excellent!  This is my favorite Email client.   Let me know if you’d like any user testing.

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There is some info about that at https://forum.emclient.com/emclient/topics/em-client-for-mac-os-x