Its not possible to retrieve deleted emails. Its true that even after deleting, copy of that email can still exist for a little longer period on the internet. This happens because email service providers have data centers at multiple places across the earth. Email copies may be made in one or more of these data centers for backup purpose, faster data transmission, etc. When you delete the email from your inbox, it will get deleted from one data center but it won't get deleted from other places unless they are updated.
Plus, as indicated above, deleted emails may be kept for a period of time by service providers.
As far as legal thing is considered, I don't think deleted emails are kept at some place for legal issues. Its possible that the email service provider keeps a record of the time, sender, etc (metadata) of the emails.