The Mailbox Importer module requires a Mailbox Importer license, a valid Client license, and proper licensing of a third-party email application that supports the IMAP or POP3 protocol.
The Mailbox Importer module's OCR functionality requires a valid Full-Page OCR license.
The Mailbox Importer module's ability to send email messages and their attachments directly to a scan queue requires a valid Document Imaging license.
Check your current licensing status by selecting Utils | Product Licenses from the Configuration module.