SMG 10.5.2-3 Problem in Sending E-mail (Recipient MX Record Issue)

We are having problem with sending e-mail to a domin with Symantec Message Gateway 10.5.2.

This customer who have multiple MX records with different priority. They are correctly resolved from SMG as matched with mxtoolbox. The host for first two MX records are not reachable and the other two remaining is fine. However, our mail is stuck in mail queue and having the following error.

451 4.4.1 [internal] No valid hosts (unable to make any connections)

I believe SMG will try sending mail in queue to lower priority MX records if the high priority ones are not reachable. But I cannot confirm this by looking at audit report. Is that anyway to check or making sure SMG has really tried to send mail to lower priority MX records?

Thanks in advance.