How can you see someones emails that they have been sending through cache?

I have an employee who has been writing nasty e-mails about me on my own computer through Yahoo E-mail. I can't log into her account because I don't know her password, but isn't there a way that Windows stores each e-mail that is sent out through cache? I found the text files but I can't open them and read them. I thought there was a way to see these files! Help! I am planning on firing this person Monday, but need this information as backup.