Legal/Litigation Hold Support (4265890)

When a reasonable expectation of litigation exists, organizations are required to preserve electronically stored information (including e-mail) that's relevant to the case. This expectation can occur before the specifics of the case are known, and preservation is often broad. Organizations may preserve all e-mail related to a specific topic, or all e-mail for certain individuals. Depending on the organization's electronic discovery (eDiscovery) practices.

Failure to preserve e-mail may expose an organization to legal and financial risks such as scrutiny of the organization's records retention and discovery processes, adverse legal judgments, sanctions, or fines.

In Exchange Server 2010 SP2 (and newer), you can use litigation hold as per Microsoft's article:

https://technet.microsoft.com/en-us/library/ee861123(v=exchg.141).aspx