Check it may be it help

Overview of Premium AntiSpam in Symantec Mail Security for Microsoft Exchange

http://social.technet.microsoft.com/Forums/en/exchangesvradmin/thread/e763de97-88a1-494d-9841-4f3a466b5604

Follow these references

Many Unexpected Pop-Ups from Symantec Email Proxy are Displayed

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!!

Hi Ambesh_444,

This message appears when sending mail intentionally from Outlook, correct?  (It is not popping up unexpectedly when the user does not even have a mail client open?)

Please ask the user to confirm 100% that Outlook is functioning fine without the SEP mail plugin enabled.

A quick internet search of that error shows several configuration conditions which can cause it... for example, http://social.technet.microsoft.com/Forums/en-US/exchangesvradminlegacy/thread/48b78890-8b54-4030-aef9-e73da7da1cb3/  

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.