SMTS-750 — Forwarder rejects mail body
Named forwarder has sent a response prior to the final dot of the mail was sent by the proxy. This behavior is correct as a security arrangement against unwanted mails.
Sending a copy of mail fails, channel is closed. Whether the proxy will try another server next time or not, depends on the response severity - servers are rotated only in case of a transient error (4xx message).