Data Loss Prevention is a set of procedures and mechanisms to stop sensitive data from leaving a security boundary. This helps you hold onto your important data and information so you do not lose it or have it end up in the wrong hands.
Data Loss Prevention is often performed on email services because this is one of the most prevalent and likely sources of data exfiltration in businesses today.
Source: Liu, S., & Kuhn, R. (2010, March/April). Data loss prevention. IEEE IT Professional, 11(2), pp. 10-13.