When you receive a document from an unknown email, you can block this email right away as follows:

In your Unfiled (Documents>Unfiled).

• Click the name of the document with the red message "Unrecognized email address" to activate the preview of the document.
• Click the "Block" button.
• The system will show you an alert letting you know that no more emails will be received from that address. Click "OK".
• A confirmation message will show. Click "OK".