Dear Sangeetha,
Here, I would like to make you aware of this kind of issue. First, confirm with your IT/Networking team and check where exactly this code has gone, from whose mail ID, and for how long. Block his/her emails for a certain time and monitor all incoming and outgoing mails.
Meanwhile, please let me know if you have policies like (1) Safety and Security, (2) Internet, Email, and Intranet Policy. Based on this, you can take the necessary action on the concerned employee.
If you are confident that the code has gone from his/her mail ID, call him/her for a discussion along with his manager, as the manager is also equally responsible for this issue. After discussing, terminate him/her with immediate effect. Warn the concerned manager on how to handle a team, and hand over this issue to your Legal team for follow-up.
I do not agree with Miss Riya, as she mentioned that if the concerned employee is good at work, he should be given a chance to improve. Giving a second chance may not guarantee improvement or trust from the manager. Instead of this, terminate the concerned employee to set an example for others to be more cautious in their work to avoid such mistakes in the future.
Suggestion: It's better to inform your IT/Networking team to get software that converts all attachments into ASCII code. This way, you can worry less in the future. Additionally, consider blocking chat facilities and personal emails for internal communications; you can use your web server for this purpose.
Hope this solution works, and all the best. Please revert for any clarification.