On the final page of the campaign scheduling wizard, when you click Schedule or Send, you can choose to either determine recipients immediately or at send time for emails. For SMS campaigns, you can only determine the recipients now so that the campaign does not exceed the estimated cost. This article goes over both options.
Determine Recipients Now
This is the default for sending in recipients' timezones and for batch sending. This option can be useful if you are sending to a very large group of people and cannot afford any delay in when the email sends. When choosing this option, be aware that people who are added or removed from a target list/segment between the scheduling and send times will not be added or removed from the group of people that the campaign sends to.
Determine Recipients at Send Time
To select this option for an email campaign, you'll need to choose a time and a timezone. Then, the Determine recipients at send time checkbox will appear.
This option is useful if you're scheduling a campaign in advance but know that your target list or segment will grow (or shrink) between now and the scheduled send time. You can plan ahead and still schedule campaigns for later without worrying about your audience becoming outdated by the time the send time rolls around.
Selecting this option means that recipients will be queued up to receive the email at send time, not in advance. If you are sending to a large group of customers, this may cause a slight delay in when your email goes out.
Note: This setting cannot be changed once the email has been scheduled and is not available for SMS campaigns.
By default, the option to determine recipients at send time option is unselected whenever you schedule a new campaign. If you would like this option to be selected by default instead, navigate to Account > Settings > Email. Here, you will see an area labeled Campaign Sending Defaults.