You see from the Message Audit Logs that outbound mail is rejected by the recipient mailserver with error 550 5.1.7 Sender Address Rejected.
You have BATV enabled on a group the sender is a member of, you have Recipient Validation enabled with the Disposition to Reject Invalid Recipients.
This behaviour is a result of an external mail server trying to verify the sender of the mail and using the Return Path as the rcpt to: when it opens an SMTP connection to the originating Symantec Brightmail Gateway (SBG). When the upstream server uses the Return Path and the originating server has Bounce Attack Validation set for the Senders Group the return path will contain a string to identify the outbound message in the event that the recipient MTA has to send a Non-Delivery Report. When this happens and the SBG has Recipient Validation enabled and set to Reject Invalid Recipients the sender verification fails which results in the recipient MTA rejecting the outbound message from the SBG.
This issue has been resolved in Symantec Messaging Gateway (SMG) version 9.5. Customers experiencing this issue should upgrade to the latest SMG release.