PLEASE HELP!

 I have being receiving many malicious emails for a number of months, I have managed to filter them as far as going to my junk and not going in y inbox. I have now got to the stage where I need to go to the police with these messages... but almost ALL of these messges have gone, just showing the ones in the last few days. I have always deleted any other junk mail I have received to leave memory room for these messages incase it ever got to the stage where I need to take them to the police.

Can you please tell me how I can retreive any old junk emails.