Add additional date-field "expire" in table mail_forwarding
With this field users can create an email aliases and define the expiration of this email-alias. At the defined date, the alias will be automatically deactivated by a cron job.
The user is creating an email-alias for a commercial winning contest and don't want to give his real email address. He knows the draw will be in 2 months. So the expiration of this email will be in ca. 3 months from now on. So he can register an alias for this and forget it.