The plugin is using the v1 version of the SDK. You should update it to use v2: - in the manifest.json, add "SDKVersion": 2, - update the common files from the Mail.app plugin. You can find the latest version here: https://github.com/elgatosf/streamdeck-applemail/tree/master/Sources/Common . What is most likely causing the plugin not to work with the SDK v2 is that kESDSDKConnectSocketFunction has been changed.
The plugin is using the v1 version of the SDK. You should update it to use v2: