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Does it require a CloudMagic third party server to operate, or it is a plain vanilla email client?

The FAQ seems to imply that having a CloudMagic account just makes it so that you don't have to set up your email accounts on each device: http://help.cloudmagic.com/article/create-cloudmagic-account...

They definitely use "the cloud", but their FAQ may be able to answer some of your questions there too: http://help.cloudmagic.com/section/security/

The FAQ is very light on details. No mention on how the mail is stored on their end

We store metadata of the last 14 days email on a rolling basis.

What about message content? Is that passed directly through or stored on your servers as well?

They are proxied through servers.