"Personal communication, whether face-to-face conversations, email, text messages, or messages shared through social media are usually run in text or given in a note. They are rarely entered in a reference list."
"When referring to communication by email or text messages, obtain the permission of the owner before using it."
Unpublished personal documents like letters, diaries, and memoirs are treated similarly.
(American Sociological Association Style Guide, 7th ed., 2022:66, 91)
Parenthetical citation:
(N. Smith, personal communication, April 8, 2020)
(N. Smith, unpublished data, May 2023)
Narrative citation:
N. Smith said in her email ....(personal communication, April 8, 2020)