Once an email has been deleted it can not be retrieved. If your mailbox is using a New mail folder, unopened mail will be deleted after 30 days. However, if you click Keep As New after opening, the mail will remain in your New mai folder and never be deleted.
Or you can switch to an Inbox folder in which mail, opened or not, is never deleted. Your can switch to an Inbox at the following url
http://inbox.aol.com/mail/changeMailboxStyle?style=self
You'll need to be logged in to mail first, however.
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