How can I delete an email that says it’s an alias

If you are talking about @icloud.com email aliases on iCloud.com, see -> Add and manage email aliases for iCloud Mail on iCloud.com -> Add and manage email aliases for iCloud Mail on iCloud.com – Apple Support (UK)

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Turn off, turn on or delete an alias

You can turn an email alias on or off at any time. You can also delete email aliases that you no longer want to use. While turning off an email alias is temporary, deleting it is permanent. When an alias is turned off or deleted, email sent to that alias is returned to the sender.

1. In Mail on iCloud.com, click  at the top of the Mailboxes list, then choose Settings.
2. Click Accounts, select an iCloud Mail alias in the list, then do any of the following:

• • Turn off an alias: select “Disable alias”.
  • Turn on an alias: deselect “Disable alias”.
  • Delete an alias: click “Delete alias”, then click Delete.

3.Click Done.

Note: You cannot delete or turn off a primary iCloud Mail address. You cannot use an alias to create a separate Apple ID, and an email alias cannot be converted to a primary iCloud Mail address.