This error can be enbountered when sending emails outbound through Email Security.Cloud, or by inbound emails through the service. A non delivery receipt (NDR) stating delivery to the intended recipient has failed.
460 too many messages (#4.3.0)
The sending email server is attempting to send more than 100 individual emails in a single SMTP connection.
This does not include a single email with multiple recipients. An individual email consists of a full set of SMTP command that specifies the sender, recipients, data and body of the mail, and the single dot sent in a line by itself that signifies to the receiving mail server that the message is completed. If the sending mail server does not close the SMTP connection, it has the option to send a new email. Doing so will add to the total of individual emails the server is allowed to send during the SMTP connection, up to 100.
The sending mail server must be configured to send 100 or fewer emails per SMTP connections.