I'm not 100% sure, but as far as I know, any web-based e-mail is only ever stored on your computer if you use an e-mail client (ie MS Outlook). If you've deleted the e-mail message, and then after emptied the online "Trash" or "Deleted Items" folder, then there wouldn't be any way of recovering the message.
I may be wrong, anyone feel free to correct me if I am, however that is the extent of my knowledge on the matter.
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