Do not include references to personal communications (e.g., letters, phone calls, conversations, text messages, emails, and interviews) in your reference list. Instead, CSE recommends placing them within the body of the paper and acknowledging them in a "Notes" section.
General Format
In-Text Citation:
(date type of communication)
Example
In-Text Citation:
...although this is not an easy thing to do (2008 letter from S Neifer to me), it is well worth the effort.