The Warning event indicates that the store driver unsuccessfully tried to deliver a message item. This event may occur if the sender of the message is determined to be invalid.

The store driver is a software component of the Hub Transport server role that delivers inbound messages to Microsoft Exchange stores. These are the databases that contain public folder stores and mailbox stores. The store driver also retrieves messages from a user's outbox, and then submits them for delivery.

To resolve this warning, use the Exchange Server message tracking feature to determine the cause of this warning. For example, search the message tracking log files for messages that are delivered to the mailbox that is specified in the event description. For information about tracking messages, see Configure Message Tracking.

If you are not already doing so, consider running the Exchange tools created to help you analyze and troubleshoot your Exchange environment. These tools can help make sure that your configuration aligns with Microsoft best practices. They can also help you identify and resolve performance issues, improve mail flow, and better manage disaster recovery scenarios. To run these tools, go to the Toolbox node of the Exchange Management Console. To learn more about these tools, see Managing Tools in the Toolbox.