You are wondering if a SMTP Email Client is installed with the Network Prevent for SMTP as you see traffic on TCP port 25.

No SMTP Email Client is installed with the Data Loss Prevention product.

The ports used by the Network Prevent for SMTP are configurable, the default ports used are 10025 for the Upstream MTA and 10026 for the Downstream MTA but we recommend using port 25 where possible. For more information on configuring the  Network Prevent for SMTP please see your Admin_Guide.pdf and Email_Prevent_MTA_Integration_Guide.pdf.

References:

Summary of ports used in DLP