You are configuring the E2CP Email Service on the first step of the Setup Wizard and are encountering this error:

The Apex Class selected is not valid. An Apex Class that implements the Messaging.InboundEmailHandler interface must be selected.

This error will occur if the ProcessMessage Apex Class which comes packaged with Email to Case Premium is not compiled before configuring the E2CP Email Service. Typically, Apex Classes will be compiled when saved. However, Apex Classes within managed packages cannot be saved, so they rely on the runtime engine to compile them when they are first used or at some point after installation.

Unfortunately, this error is related to a platform limitation and is beyond our control. To avoid this error, you will need to manually compile all Apex Classes in the Org.

To compile all Apex Classes in your Org:

1. Navigate to Setup > Develop > Apex Classes.
2. On the top left of the page, click the link labeled "Compile all classes".
3. Once successfully completed, you should be able to configure the E2CP Email Service without error.