You do not have to list memos in the reference list according to the American Psychological Association. Memos are considered personal communication and should only be cited in the text of your document. The information should be formatted as follows: I. Harris, personal communication, September 16, 1996.
MLA requires memorandums to appear on the works cited page. The citation should be include the name of the writer, the recipient of the memo and company or organization receiving the information. Additionally, the day, month and year of the correspondence should be mentioned. The entry should be organized like the following example: Smith, M. Memo to Fine Arts dept. faculty, Southeast Tennessee College, Memphis, TN. 9 September 1989. The in-text citation should include the writer's last name in parenthesis. For example, (Smith)