Missing Organization Feature: Email2Case

The "Email Message" standard object is created in a Salesforce organization when Email to Case (standard) is enabled. If you are receiving this error, it is because you are installing an application that has a dependency on the Email Message standard object and its not available.

To resolve this, simply enable Email to Case (standard) by navigating to Setup > Customize > Cases > Email-to-Case. Only the first checkbox labeled "Enable Email-to-Case" needs to be checked. It is not necessary to make any other configuration changes there.

Rest assured that checking "Enable Email-to-Case" will simply create the standard Email Message object in the background and no visible changes will appear for end users.

This error is expected and by design when installing a package into a Salesforce organization with a missing dependency.