In Symantec Messaging Gateway (SMG) Invalid recipients are not getting rejected and upon further investigation it can be seen that it´s because all the inbound mails are seen as outbound in the Message Audit Log. Since Recipient Validation only works on Inbound mail direction no mails are getting validated although it´s been confirmed the Recipient Validaion is configured correctly and when testing individual email addresses the correct result is returned.
Outbound mail acceptance has been specified incorrectly. Since upstream mail hosts (smtp gateway/relays) has been added into outbound mail acceptance instead of internal mail hosts a loop is created and the SMG will not detect the mail direction correctly.
Remove the incorrect mail hosts from the outbound mail acceptance and add them to inernal mail hosts instead. Ensure that only the downstream mail servers such as the Exchange server is specified here. Once the SMTP settings are set up correctly inbound mails will be seen as inbound and Recipient Validation will work correctly.
Symantec Messaging Gateway (SMG) 9.5