There is a feature in Marketo to limit the maximum number of people that can qualify for a smart campaign. This avoids accidentally emailing your whole database.

 

This applies only to batch campaigns and email programs.

Click Admin and then Smart Campaign.
Click Edit.

 

If the number of people that qualify to run through a smart campaign exceeds the limit set, then it will not run at all.
 

Enter a limit and then click Save.

 

Disable this feature by making this field blank. 
This limit is applied to all the smart campaigns but can be overridden at campaign level. Learn how to override person restrictions in a smart campaign.

Bingo! You just turned on the safety switch. Good idea, right?