If you are using Email Security.Cloud (ESS) you are protected from Petya ransomware. All Symantec email customers are fully protected from Petya with our latest released set of signatures. Our Skeptic and Link Following technologies available in our Email Security.cloud product will provide additional proactive protections.
In addition, our Symantec Email Threat Detection and Response technology would’ve blocked such of emails containing malicious attachments due to our sandboxing capabilities via virtual/physical environments.
Please see link for more information: Petya ransomware outbreak
We also recommend implementing the steps in our Email data protection articles if possible. This is for customers inquiring about Ransomware and the best way to use email data protection to protect against specific file/MIME types as these vectors are still being used by other ransomware attacks.