For any training, first establish the need. For example if you are seeking participants for a program on email etiquette, get sample amils from your potential participants and share them with your boss and point out the mistakes without taking the participant name. After that come out with a program to fill the gap and after the training and a few hand holding sessions in a virtual mode, ask their managers if they are better in their email writing. If they have to write to external people like customers, they cannot write emails in a wrong way. That itself is justification. Similar is the case of customer sevice, presentation skills, interpersonal skills, leadership skills etc. Internal trainer is at an advantage in finding the problem, though actual trainng can be by either internal or external based on whether adequate expertise is available internally or not.