Some Anti-Virus software products (e.g. McAfee starting with version 8.x) block and/or intercept outgoing connections to port 25. This will interfere with the EventSentry SMTP notification target which sends emails using SMTP port 25.
You will need to disable or customize the SMTP protection feature of your anti-virus product to make the SMTP notifications work.
In McAfee you will need to disable the feature to prevent mass-mailing worms (in "Access Protection") from spreading, as this feature blocks all outgoing SMTP connections. You may also be able to edit the McAfee rule to exclude the EventSentry service (eventsentry_svc.exe) from being blocked.