E-mails and letters not available to readers are treated as personal communication. Since the information from personal communication is not available to readers you do not include it in the reference list but only in the text. Personal communication also include non-recorded lectures, text messages, chat messages or direct messages, interviews, speeches etc.
Only use personal communication if another more available source is not available. In the in-text citation you write initial and surname and a date (as specific as possible).
Parenthetical citation
This is due to... (A. Andersson, personal communication, October 25, 2017).
Narrative citations (when the author is mentioned in the text)
According to A. Andersson (personal communication, October 25, 2017) this aims to ... .