I've created my own implementation of WorkflowMessageService.
When I used the Sendinblue plugin in overrides SendinblueMessageService : WorkflowMessageService.
Sendinblue only overrides a single method "public override async Task<int> SendNotificationAsync".
I suggest the "public override async Task<int> SendNotificationAsync" method be moved to another service so that we can implement custom WorkflowMessageService but still use Sendinblue to actually send the email via api.