Symantec Messaging Gateway Mail Flow Error

Dears,

I have a problem with the inbound mail flow after i have installed SMG virtual Appliance.

i have an Exchange Mailing Servers 2013 with 2 mailbox and 2 CAS servers with Microsoft Network load balance between the 2 CAS Servers.

when i change the nating rule on the firewall to forward traffic to inbound interface of the SMG and try to send an email form an external mail server to my Exchange i got this error

error transferring to mail.*****.com; SMTP Protocol Returned a Permanent Error 554 5.1.2 Recipient address rejected: User unknown (Transfer failures occur when there is a transient connection failure between the servers)

any help ?