How to Configure Packet Monitor for SMTP Traffic
Go to Dashboard/Packet Monitor/Configure button

Settings Tab
Check Exclude Mgmt Traffic: HTTP/S
Check Exclude Internal Traffic: HA and SonicPoint
Monitor Filter Tab
Ether Type: IP
IP Type: UDP, TCP
Destination Port: 25
Check Enable bidirectional Address and Port Map
Display Filter Tab
View all: check all
View items: check all
Advanced Monitor Filter Tab
Check all items and click OK
select Start Capture