E-mail messages are treated as personal communications, and because they do not contain recoverable information, they are not included in the reference list.
See the explanation on the APA Style website for more details:
All information and page number references are based on the 6th edition of the Publication Manual of the American Psychological Association (2009/10).
Need a copy of the Publication Manual? Click here for details. See Also: APA Style Guide to Electronic References, Sixth Edition