How do I cite email correspondence in APA style?

The American Psychological Association answers many questions like this in their FAQ section on the web. For example, they have a link explaining how to cite emails from an individual:

How do you cite e-mail communications from individuals?

E-mail communications from individuals should be cited as personal communications. Because they do not provide recoverable data, personal communications are not included in the reference list.

Cite personal communications in text only. Give the initials as well as the surname of the communicator, and provide as exact a date as possible.

 Examples:

  • T. K. Lutes (personal communication, April 18, 2001)
  • (V.-G. Nguyen, personal communication, September 28, 1998)

It is possible to send an e-mail note disguised as someone else. Authors—not journal editors or copy editors—are responsible for the accuracy of all references, which includes verifying the source of e-mail communications before citing them as personal communications in manuscripts.

(adapted from the sixth edition of the APA Publication Manual, © 2010)