An outbound message is not delivered and the sender receives a Non-Delivery Receipt (NDR) from Symantec Messaging Gateway (SMG) giving "554 5.4.4 [internal] Domain Lookup Failed" as the reason for delivery failure. A look at the delivery queue before the failed message has triggered the NDR, may also show the "454 4.4.4 [internal] no MX or A for domain" error associated with the failed message in the delivery queue.
This error indicates that SMG could not get an MX record or an A record of the recipient domain when performing DNS lookup.
This could be due to:
Ensure that the recipient domain is a valid domain, and that MX and/or A records exist for that domain.
If the domain is valid and has MX and/or A records, then make sure that the DNS server configured on SMG is able to resolve the domain correctly.
If the problem is intermittent then the configured DNS server may be having issues due to load or some other network problem.