Dear Sangeetha,
Here i would like to make you aware on this kind of issues,
first make confirm with your IT / Networking team and check where exactly this code has gone and from whose mail id and for certain time block his/her mails and do monitor all his/his mails coming in and going out.
mean while do let me know, do you have the policy like (1) Safety and Security, (2) Internet, Email and Intranet Policy....basing on this u can take the necessary action on the concern Employee.
Be confident that the code has gone from his/her mail id and then call him/her and discuss with him/her along with his Manager, because his/her Manager is also Equivalent responsible for this issue...
after discussing Terminate him/her with immediate effect and warn the concern manager how to handle a Teamand hand over this issue to ur Legal team and do follow up with your Legal team....
Here i dont agree with Miss .Riya, as she was saying that if the Concern Employee is gud in working need to be given a change to make changes of himself, if you given a chance again what would be the use will he be perfect and will their concern manager can trust him again and can allow him to work in his team, instead of this u can terminate the concern Employee so that by seeing this way of terminating other employees can be more caution on their work and this kind of mistakes will not hapen again...
Suggestion: Its better for you to inform to your IT / Networking team to get the software which will convert all the attachments into ASCII code, by this way you can be worry less in future also....and even block the chat facility and personal mails too
for internal communications u all can use ur web server...
Hope this should work and all the best
Revert back for any clarification