

No, I mean their email server + protocol.
The thing you’d use to self-host a proton mail email server.
What you linked is the source to the client,
which interacts with the proprietary server code to fetch your mail from them.
If you can’t self-host / switch to a different server if they enshittify due to being closed source, then it’s not “open source” nor “portable”
Sure the client is open source,
server code is closed source.
So if proton enshittifies or ceizes to exist,
the open source clients will be useless,
since you or someone else can’t host the server, since that is closed source.
But whatever man, good for you if you like them and want to remain oblivious of the risks! c: