NXDOMAIN can be a temporary error. The SMTP queuing protocol is designed to be resilient against DNS failures, internet outages, routing problems, and temporary mail delivery issues.
Unless some DNS server is broken, it actually cannot. NXDOMAIN is an authoritative answer that tells you that the domain positively does not exist. Not to be confused with SERVFAIL, which you should get if the DNS resolver ran into a timeout or got an unintelligible response or whatever, NXDOMAIN should only occur if the authoritative nameserver of a parent zone explicitly says "I don't know this zone either locally nor do I have a delegation for it".
