Outbound is a service that allows you trigger messages based on the actions your users perform. Once integrated, you'll have access to a dashboard from which you can create and monitor your different campaigns.
Unlike some of the other campaign tools available through Segment, Outbound allows you to send message via email, push notification, and SMS. You can also set goals for follow-up actions and run A/B tests on different message texts and different modes of contact.
You could use this tool to send a customized email to new users who have just signed their first campaign. Or you could use it to send a text message to users who created a petition 10 days ago, reminding them that they can collect signatures on paper and email their supporters.
Another service that lets you send emails and drip campaigns to users who have taken certain actions is Customer.io.