Answer:
When citing an e-mail, this is the format we should use in the works-cited page:
Last name, First Name. "Title/Subject of the E-mail." Received by recipient, day month, year.
1. In the event that we are making a section for a works-refered to list about the principal email that was sent by Kari, it would be this way:
Ridge, Kari. "Methods for Storing Nuclear Fuel." Received by Davis Hamletter, 17 April, 2009.
Note: the subject of the e-mail (its title, so to speak) was not mentioned in the information provided. But, since we know the e-mail is about "methods for storing nuclear fuel", I used that as the title.
2. Now, if we are creating an entry for the reply sent to Kari, it would be like this:
Hamletter, Davis. "Re: Methods for Storing Nuclear Fuel." Received by Kari Ridge, 17 April, 2009.