You obviously must trust that the server runs this configuration, but you can always run your own chatmail, or regular postfix. (If you don't need "federation" with other mail servers, you don't even need port 25 open).
However, you can also configure your app to delete messages from the server sooner.
https://github.com/chatmail/relay/blob/96a1dbac08441034c5990...
https://github.com/chatmail/relay/blob/96a1dbac08441034c5990...
You obviously must trust that the server runs this configuration, but you can always run your own chatmail, or regular postfix. (If you don't need "federation" with other mail servers, you don't even need port 25 open).
However, you can also configure your app to delete messages from the server sooner.