Use the communication’s citation form

Warning²Federal courts, Illinois courts21st and 22nd editions

A letter or memorandum names its writer and addressee, a listserv posting both e-mail addresses, and an in-person interview its location; each says where a copy is filed.¹

1. 1R. 17.2 Unpublished Materials; R. 17.2.3 Letters, Memoranda, and Press Releases; R. 17.2.4 E-mail Correspondence and Listserv Postings; R. 17.2.5 Interviews.
2. 2A warning: the form is usually wrong under the rule in note 1, and context can justify it.