Evidently, the US Army Captain's Career Course, which I understand is required before promotion to that grade
Close, but not quite.
Students at the course range from 1st Lieutenants who are very close to promotion to mid-grade Captains (2 years or even more time in grade).
This is especially true for reserve component officers (Army Reserve & Army National Guard). This is due to funding levels that can change annually, number of slots to the school, ability of officer to attend due to work, family, etc.
... does not cover e-mail etiquette. Maybe in the future it will?
I really doubt that they have time to add that to the curriculum. Yeah, it would only be a 1 hour block, but it probably would best be handled through many of the informal start-of-day discussions that happens in the small group phase of the course. Assuming they still have a small group phase portion of the course.
I went to the resident active-component course as a Army National Guard officer many, many years ago.