Deleted emails are sent to the Deleted Items folder located in your account's folder list. You can easily restore an email from the Deleted Items folder by dragging it back to the folder of your choice.
If a deleted email cannot be found in the Deleted Items folder, then this email was likely purged from the folder at some point. It is possible to recover emails purged from Deleted Items under certain conditions. Please reference these articles for recovery instructions:
Emails purged from Deleted Items more than 30 days prior are irrecoverable. The Help Desk is unable to assist with recovery of email beyond a 30 day period.