Email recovery

See the articles Delete email in Mail on iCloud.com - Apple Support

Delete and recover emails in Mail on iPhone - Apple Support

Spdiesel Said:

"Email recovery: I lost a very important email from my iCloud email. Can anyone please help. Thanks"

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Retrieving Via eMail:

If you forwarded it off to somebody, ask the recipient to reply back to you with the message. You'd have the contents.