RCode2 (ServFail) is more interesting than RCode3 (NxDomain) which just indicates someone asked for a non-existent name. #DNS

SERVFAIL covers a multitude of errors, including:
- an expired zone on an authoritative secondary
- a DNSSEC validation failure
- other failures to resolve a domain name (e.g. broken delegations)

None of these indicate a "server" issue, as such. They are all protocol issues.