When Iterable API calls fail across the board, check which data center your account lives in. USDC and EUDC are separate instances with separate API hosts, and pointing at the wrong one fails everything. And every call needs at least one of email or userId: without an identifier there is nothing for Iterable to attach the event to.

Context: RudderStack's open-source Iterable destination documentation FAQ: USDC and EUDC are geographically separated Iterable instances, and you must choose the data center matching your Iterable account region. Data cannot be shared between data centers. It also states that either email or userId is mandatory on every Iterable API call; without at least one identifier the destination throws.

## Matched source
Source: Published skill
Original query: "Iterable calls fail everywhere: wrong data center or missing identifier"
Key terms: calls, center, data, everywhere, fail, identifier, iterable, missing, wrong