Add support for Gmail accounts (external mail accounts)
Hi Proton, I love using Proton Mail but there is no getting away from using my Gmail account also.
I would love it if you could add desktop/web (and mobile) app support for Gmail accounts, so I can use just 1 email client (Proton Mail). It is perfectly fine, and expected, that you will not be able to offer the same security and encryption for Gmail as Protonmail. The value of adding this support is in the convenience.
I have spent a ton of time trying to make Proton Mail work with other clients like Spark, Airmail, and Canary (nevermind Superhuman) and while I can import and read them, none of the productivity features work well or at all. And, I'm tired of duplicating my organization efforts and managing to inboxes.
Thanks so much for all the great features you already added recently. I hoping for a few more. :)

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Kava Blanca commented
The free Mailvelope browser extension can be used to add PGP encryption and digital signing (certified email) to Gmail, Outook.com, and Yahoo web mail.