Is SMTPS (implicit SSL) used to secure DLP Email Prevent traffic between Exchange > NPE Detection Server > SMG?
Currently, DLP does not support SMTPS.
There is an existing feature request (FR) for adding support for SMTPS.
That FR is targeted for a future release.
If you want the ability to use SMTPS, open a case and ask to be added to the FR, and reference this KB.