(ASA Style Guide 2014 section 5.3.6 p. 85)
In an e-mail message to the author, (Last name) indicated (subject).¹
In an e-mail message to the author, Smith mentioned that the association plans to make this change.¹
¹Sender first last, e-mail message to author, month day, year.
¹John Smith, e-mail message to author, July 6, 2017.
Email messages are not included in the References section at the end of the paper, so they only receive an in-text citation with a superscripted number for the note, and then a footnote at the bottom of the page with the full footnote citation.