You are sending an email to several hundreds recipient using the Symantec Email Security.Cloud outbound scanning service, or are sending to a customer using the Symantec Email Security.Cloud service.
452 too many recipients (#4.5.3)
You are sending an email to more than 500 recipients in a single SMTP transaction. This means that in a single transaction, the number of recipients specified by the RCPT TO command exceeds 500. For each addition RCPT TO command issued, the Symantec Email Security.Cloud server responds with "452 too many recipients (#4.5.3)".
Configure your email server to send to a maximum of 500 recipients within a single email.