EU VAT guide
VIES errors explained
VIES errors are usually not final tax conclusions. Many are temporary gateway, member-state, timeout, or rate-limit issues.
Temporary errors
SERVICE_UNAVAILABLE means the VIES gateway is unavailable. MS_UNAVAILABLE means the national VAT registry is not replying.
TIMEOUT means VIES did not receive a reply within its allotted time. Retrying later is usually the right next step.
Traffic errors
GLOBAL_MAX_CONCURRENT_REQ and MS_MAX_CONCURRENT_REQ mean VIES or a member-state registry is receiving too many concurrent requests.
A production integration should use backoff, queues, and careful per-country concurrency limits.