Hello,
If subject lines and recipients are displayed, examine them to determine if mails were intentionally sent from the mail client.
If not, isolate the computer from the network and follow best practice to determine if a currently undetected threat is operating on this computer. Checking what program is using common mail ports (performing a netstat -ao from the command line to learn what process is communicating on port 25) is often the best first step.
Please see these two articles that address this issue. Let me know if you still have questions.
Many Unexpected Pop-Ups from Symantec Email Proxy are Displayed
http://www.symantec.com/business/support/index?pag...
Unexpected Pop-Ups from Symantec Email Proxy
http://www.symantec.com/business/support/index?pag...
Chances are that the computer is infected and is sending out spam. I recommend putting the very latest definitions onto that computer, isolating it from the network, and performing a full system scan in safe mode.
Also, check this Thread: https://www-secure.symantec.com/connect/forums/symantec-email-proxy-rejection-intermittent-100310
Hope that helps!!