On the final page of the campaign scheduling wizard, when you click Schedule, you will see an option that reads: "Determine recipients based on members of selected list(s) at send time instead of right now."
NoteThis setting cannot be changed once message has been scheduled.
This option can be 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 You can plan ahead and still schedule campaigns in advance, but don't have to worry about your audience becoming outdated by the time the actual send time rolls around.
Selecting this options 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.
Account Level Setting
By default, the 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, you can configure this in your account settings by navigating to Account > Settings > Email Settings. Here, you will see an area labeled Campaign Sending Defaults.