What about emails as evidence? Can you retrieve emails that have already been deleted from an account?

On a home computer there’s no such thing as deleted.  It’s there someplace.  You just need somebody to come in to be able to find where it is.  A computer forensics person can come in and they’re normally charging anywhere between $250 and $450 an hour and they’ll take the hard drive, they’ll make a full copy of the hard drive, and they’ll take it back to the lab, and they’ll go through all of the sectors, and especially those that have been the deleted sectors, and they’ll be able to find all hidden chat room conversations, instant messaging, e-mails.  So, it’s never gone unless it’s been properly wiped, and most people don’t know how to properly wipe a computer file.

You can.  You just have to have the right knowledge, and experience, and software to do it.  So, just pressing delete doesn’t delete anything.  It just makes it delete off of what you see on the screen, but it’s actually still in your computer.