In the body of your text, you need the following: Initials and Surname of communicator, and provide an exact date as possible.
Examples
... has been recently identified (May 16, 2019 e-mail from RS Stewart to me).
... the effect on paediatrics (Jun 6, 2019 letter from P Gluckman to me) including the scientific significance.
If you are making a direct quotation, use quotation marks.
J Cook (Apr 1, 2019 email to me) stated that "the chemical compound had ionized".