PDF invoice copy not sent to the customer

Hi Experts,

Please help me to resolve issue in below scenario.

The customer invoice is created in VF01 and then a output is triggered by which a PDF invoice copy is sent to the customer by email .Today customer did not receive the PDF invoice for one spool. I am not sure if this spool was sent to the output device or not . User says email server was properly working today . All these steps are executed automatically through background jobs.

Thanks in advance !