What is the difference of between an e-mail and a work item?

E-mails and work items are two completely separate things. They just happen to be displayed in the same inbox. An e-mail is a message sent to one or several people. However, a work item is a runtime object of a single-step task or workflow. Consequently, a work item cannot be deleted from the inbox of a user. In this case in fact, you have to adjust the agent assignment or delete the work item as described in note 49545.