Go to your victims computer, open up their e-mail program, select every message they have in their In-box and click OPEN.
If this person has around say, 1300 messages in their In-box, the e-mail program is going to try and open every damn one of these messages. But unless they have their e-mail program set up to handle opening 1300 messages at the same time, the program is going to bog down into a horrible mess. It will take minutes or even hours to open all of those messages.
But you are not going to let it open all of those messages.
No, you see what you will do next is force quit the e-mail program. Some programs you can only quit, but either way here is what happens. The program will remember that the last time it was opened, it had 1300 messages open and it will go through the process of opening every one of those 1300 messages until the user either manually closes all of the messages or gets smart. Smart in this case is to close out of the e-mail program, reset the preferences and then open it.
Resetting the preferences will reset the e-mail program and this might not be something that can easily recover every message or mailbox.