Some messages are being rejected by the destination mail server and a review of the Messaging Gateway (SMG) message audit logs (MAL) indicates that the email messages were rejected with the following SMTP response code:

550 5.7.1 sender id (pra) not permitted

This is usually an indication that the message failed either SPF or SenderID sender authentication and that the sender address in the email From header, otherwise referred to as the Purported Responsible Address (PRA), is not allowed to send from the SMG IP. 

Please confirm that the SMG IP addresses are listed in the DNS SFP or SenderID records as an authorized sender for the message envelope sender or the email domain used in the message's From: header.