Support multiple accounts on iOS
Either allow multiple accounts to be open simultaneously in the iOS app or add option to switch between accounts without having to formally log out and log in between each swap.
Despite allowing multiple aliases for a single account, if you have a personal account but also shared an account (eg with a spouse) aliases don’t suffice. The idea of forwarding every email from one account to another is archaic, lose for user and lose for proton mail because you’ll be storing the same message twice unnecessarily.
Multiple account support is a basic feature for email apps.

Thank you for your feedback! The iOS Proton Mail app supports multiple accounts.
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ANDREAS commented
I would like to be able of switching between protonmail accounts in my web based accounts and in my mobile app as well. A very exciting feature for all users, like instagram or other simillar apps already are doing so.