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Invalid request: missing email or domain, empty items, more than 10,000 items, or a malformed body.

Missing or invalid API credentials.

Too many requests: the platform cap refused the request, or it was rate limited.

A platform cap refusal has error set to platform_cap_exceeded. Accounts have a daily and a monthly usage limit per meter, and kind says what retrying can achieve:

• limit_reached: this request would take usage past the limit. The same request succeeds once the limit resets at resets_at, which Retry-After gives in seconds.
• request_too_large: this request alone is larger than the limit, so retrying cannot succeed. Make it smaller (fewer items, or a smaller page_size), or contact support to raise the limit.
• shut_off: usage is paused for this account. Contact support.

message says the same in a sentence you can show your users.

Any other 429 is rate limiting: wait for Retry-After when it is present, then retry.