Those are called “Drip” campaigns. The marketer sets up a series of email campaigns and sets them apart at timed intervals. After a “trigger” of some sort (like when you sign up for something), the emails start dripping into your inbox.

When expectations are set, and when the content is actually useful, drip campaigns are really powerful and cool.  But if you don’t set expectations, it can be like water drip torture.

Here’s an example of properly setting expectations for a drip campaign (click to zoom in):