Personal Communication includes: private letters, memos, some electronic communication (i.e. email or messages from nonarchived discussion groups), personal interviews, telephone conversations, etc.
Although these types of communication are not recoverable data, they should be included on the Reference page.
Example:
Author. Affiliation, City, State. Phrase describing the material, Year.
Feynman, R. California Institute of Technology, Pasadena, CA. Personal Communication, 1985.