When configuring the Enforce Server to send email alerts, you might get the error shown below:
Extract from the Release notes:
As of version 15.0, Symantec Data Loss Prevention no longer supported anonymous logins to the SMTP server that is used for sending out alerts and reports. You must enter a valid user name and password.
For versions 15.0 - 15.8 MP1: Don't use anonymous login to the SMTP server that you use to send out alerts and reports. Simply input the Authentication information at your SMTP Server (User ID and Password required now).
Also, if you are using Microsoft O365 email, make sure that DLP Enforce server is allowed in O365 to send email.
This issue is now fixed in DLP 15.8 MP2. Customers can use anonymous authentication to connect to SMTP relay.