Is it illegal to open another person's email?

It depends on who it is and how serious the e-mail is. If it is a sybling/ parent or other family member as a simple joke (nothing serious) then you aren't being sent to jail. If you are using a boss's or someone you despises e-mail to mail someone a virus or other destructive item then yes it is and you are definitly getting arrested (so don't try it because even if it is another e-mail they can track the ip address it came from to track down all of your personal information and the cops will end up showing up at your front door.)