This document explains whether Service Desk r12.7 support POP before SMTP as an authentication type for the SMTP mail service.
No. Service Desk r12.7 does not support POP before SMTP.
Prior to Service Desk sending out Notification emails, it does not check for incoming emails, i.e. POP3 authentication does not occur prior to sending emails. So if the SMTP server requires POP3 before SMTP authentication, notification emails may fail to be sent out.