There are several ways to delete mail in the Mail app of macOS High Sierra. To delete a message from a Mail notification or with a swipe, the “Move discarded messages” option in Viewing preferences in Mail must be set to Trash; otherwise, the message is archived.
- In a Mail notification: Move the pointer over the notification, then click Trash.
- In the Message viewer: Do one of the following:
- Swipe left with two fingers, then click Trash, or swipe all the way to the left until the message disappears.
- Move the pointer over the message header, then click the Delete button (it looks like a trashcan) that appears.
- Select a message in the list, then click the Delete button in the Mail toolbar.