If you want to send Email Marketing to all of your contacts that have an 'Opted-in' subscription status, you can send an Email Marketing piece to 'All Contacts' and just make sure to de-select the option to 'send to email addresses with an 'Unknown' subscription status'. Therefore, it will only go to contacts who have 'Opted-in' status.
Note about 'Unknown' Subscription Status: your Contact's email subscription status will be automatically set to 'Unknown' when they are added to the Keela system without an email subscription status. This means that the Keela system has not received direction from either the Contact or your organization about what sort of status they should have (ie. Opted In or Opted Out).