An email should always include an unsubscribe link where they can choose to no longer receive email communications from you.
Another, more user friendly, option is to insert a subscribe link. This lets the user feel more in control of their choice and is less blunt.
- Highlight the text you would like to make your Unsubscribe Link.
- Select Insert Unsubscribe Link from the right hand side of the Editor.
As with any email and its features, please make sure to thoroughly test any links before sending.