Say your organization is doing something like Amnesty International (at least sounds awlful lot similar to me), you want a solution that
- encryption
- shared inbox between trusted members
- minimal meta-data leak to providers (service providers and network node operators)
- hide who is sending/receiving
- easy to search/indexed
- fine grained access control
- audit log of who responed to who
- multi-device
- single stable address/contact point (how “stable” you need it to be?)
- 2fa?
- easy to use
Am I correct? To be honest, it is quite a tall order. I can’t really think of a solution right now. Email is definitely out of the question because you can’t hide who is sending and receiving the email.
This is a reality of any software. Those requirements exists by themselves or in some combinations, but once you want them all, the difficulty grows exponentially.
The Sunbird model works. Their model isn’t that hard to replicate, and have the steps laidout for you to copy. However, it doesn’t offer some perks you want with limitations. For example, you can only have 5 devices linked to 1 Signal account. There is no 2FA, fine grained access control, nor audit log. The search functionality is not particularly good.
There are ways to overcome those limitations but you will need some tech savvy dude with proper security backgroud/training to design, implement, and manage that. It steps into semi-custom developement and integration, and be warned, it is hard to done right, especially anything with security.