DOC6514: ServiceDesk 7.5 MP1 User Guide
Chapter 13: Creating incidents from user emails (page 163)
Practically, what often gets overlooked is that for immediate incident creation, e-mail subject needs to contain "New Incident" (it is mentioned in Table 13-2 How emails are processed for incident creation).
If that string is not in the subject, a new kind of process is created - EM-00xxx (SD.Email.InboundManagement) for classifying these inbound e-mails. These are by default shown (or more accurately, assigned) to users in Service Managers group.
Settings for inbound e-mail mailbox are located in ServiceDeskSettings - Admin > Data > Application Properties > ServiceDeskSettings - Category: Mail Monitoring Service.