Wow, the ProtonMail Bride is pretty neat!

I'm not affiliated with Missive but I thought I'd add my 2 cents. Since ProtonBridge assumes the Mail client runs on your computer (along with the bridge) and grabs mail over IMAP and send over SMTP it isn't likely that Missive could support this product. Missive is a cloud-based mail client (like GMail), which means that the client is just downloading a view/copy of your mail for view in the Missive web-app (mobile is similar). It doesn't send/receive mail over those protocols (instead, HTTPS) from your computer, but rather from Missive's servers.

I'm not saying this is impossible but it's not a low-medium level of effort either. I look forward to Philippe Lehoux response too :)

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The ProtonMail Bridge is an application for paid users that runs on your computer in the background and seamlessly encrypts and decrypts your mail as it enters and leaves your computer. It allows for full integration of your ProtonMail account with any program that supports IMAP and SMTP such as Microsoft Outlook, Mozilla Thunderbird and Apple Mail.