There's no good way around that unfortunately. The proxy could build in an OAuth client for the major providers, but it's unlikely that this would be trusted by default without significant effort being put into review processes.
As the readme explains, there's nothing to stop you using the existing OAuth client details from another source (such as the many already trusted open source email clients that exist).
As the readme explains, there's nothing to stop you using the existing OAuth client details from another source (such as the many already trusted open source email clients that exist).