You need a way to identify which Email Listener picks up the email which creates an Incident.
This functionality was added in the 2014.1 release.
- Go to the Inbox workspace
- Open your Inbox record
- Make sure 'Email Processor' is set to 'Incident'
- Check the box called 'Include Source Mailbox Name'
- Click Save
Now when an Incident is created from an incoming email, the 'Created by' field will show the name of the mailbox.