Non-personal, shared (aka functional) mailboxes are commonplace in businesses - a central mailbox for a particular purpose accessible by multiple users. A similar need is also common for personal use (e.g. each spouse has their own personal mailbox, but also have a shared address for things like bills or school notifications)

1) Best option would be to allow an address to be created and assigned to multiple users within a ProtonMail Organization. That avoids forwarding of emails and also avoids having to log out of one ProtonMail user and then log into another user in order to check the shared addresses.

2) A potentially easier alternative would be to allow simultaneous login to multiple users via the Protonmail client. Thus you could login to each user account once and easily toggle between personal vs. shared mailboxes.

3) In the meantime, could you at least add an additional Action to the Filtering function that would allow you to auto-forward an incoming email if it met the defined conditions. This is not ideal as it essentially duplicates the data and the "other" user won't have replies in their Sent folder, etc. It's messy but better than nothing.

Thanks