An update for the ProtonMail Bridge app offers to encrypt and decrypt emails that are stored locally for much quicker access. There's a new Status View and stability improvements as well.
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Proton Technologies has released the ProtonMail Bridge app for Linux. Bridge allows you to use a desktop e-mail client to access your ProtonMail account. It's already available on macOS and Windows.
The team behind ProtonMail has released ProtonMail Bridge for Windows and macOS with plans for a Linux release in spring 2018. It allows you to access ProtonMail through a desktop mail client.