The Apple Mail client connects to your email Web server to import messages and store them on your computer. If you accidentally delete a message, then it is likely that the email can still be found by logging into your webmail. Additionally, you can use Apple's Time Machine feature to restore a backup of your email.
Check Your Webmail
Visit the website for the service that hosts your email, such as Gmail, Comcast, or Microsoft Live. Sign in to your webmail account and check categories such as "All Mail" and "Trash." Depending on your email account and Mail client settings, messages deleted in Apple Mail might be filtered to different directories within your webmail account.
Restore from Time Machine Backups
Launch the Apple Mail client and click on the Time Machine icon in your system toolbar, which looks like a clock surrounded by a circular arrow. Select "Enter Time Machine." Choose a backup date from the timeline. Examine your mailboxes to make sure that the deleted messages are present, and then click on "Restore."
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