If there is a maximum limit configured for the number of email recipients at the smart host, the DLP Prevent sends a 4xx error after the DATA part of email to the client edge server. The reason is because the DLP Prevent system tries the onward delivery ONLY after it receives all RCPTs from the client server. But, after the smart host reports 4xx for a particular RCPT TO, the DLP Prevent replies to the client with 4xx after the DATA part. Some clients like Edge server might not work with this sequence because they expect 4xx after the RCPT TO reaches the maximum limit. The email thread is lost.
Based on the protocol logging on the Edge server, the DLP server sends the 4xx and
QUIT command to Edge after it receives the
452 4.5.3 Too many recipients error message from the smart host. The client might not expect this response. The response impacts all recipients in the session, that is, the recipients other than 500 also fail to receive the email.
The internal hub transport server message tracking log confirmed that all users in that SMTP session failed to receive the email that was delivered to them.