Customer Having problem with End users which are continuouly sending unknown mail from their system?

Customer Having problem with End users which are continuously sending unknown mail from their system?

They are using Symantec Endpoint protection 12.1.3 . Has 300 users in the system and they complain me from 10-15 PC lots of mails is sending automatically which are not send by the users. So what kind of solution I will provided to them. Any suggestion you can give to me!?