Depending on how your email server handles mail, your should be able thru your browser, to go to your email account and see if those emails are still there. If so, then by following the link at the top. You can delete your mail account and then re-add it. which should cause your email provider to re- download the mail. Or you can do it by using the rebuild function in the mail app. Which is under mailbox in the top menu. If they are not on your providers website in your mail account., then I know of no way to retrieve them.