Unsent mail is stored in your drafts folder, or your outbox depending on your email client. To recover the drafts or outbox folder, and the emails within it, you can import your most recent email backup file. If your last backup file was created before you wrote your unsent email, it won't be in there. If you accidentally deleted your unsent email, it's probably still available in your hard drive's "dumpster." Outlook has a built-in recovery program that can recover emails from the dumpster. Commercial software is also available to recover email files.
Open the "Deleted Items" folder in your email program to see if your unsent email is still there. If it is, just drag it to the inbox or other folder to recover it. If not, try the built-in dumpster recovery program.
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Click on the "Tools" menu to find out if your email client has a file recovery program. In Outlook, click on "Recover Deleted Items...." You can also search "Help" to find out if your program has a similar option with a different name.
Look through the list of deleted items that appears. Select the unsent email you want to recover and click on the "Recover" icon above the list. If you don't see your email file listed, you may need to try commercial recovery software.