AR 623-3 paragraph 3-6 states: The rater uses DA Form 2166–8–1 to prepare for, conduct, and record results of performance counseling with the rated NCO. Its use is mandatory for counseling all NCOs, CPL through CSM.
Counseling is supposed to be conducted face-to-face unless there are extenuating circumstances. If the rater wrote “See attached email dated DATE. Discussed content of emails with NCO and…” on the DA Form 2166-8-1, then perhaps he could support his argument to a degree. However, I do not believe he would be able to support an argument that he conducted counseling solely through an email. This would be a call the chain of command would have to make based on all the facts.
Read The Mentor: Everything you need to know about leadership and counseling for more information about Counseling, Leadership, Corrective Training, and Separations in the Army.