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The customer-facing message stays generic on purpose (never leak Brevo's internal error text), but the real reason was discarded entirely before this — every failure looked identical from the outside. Cost real debugging time today tracking down a misconfigured BREVO_LIST_ID in Coolify (was "2", should have been "5") that made every single signup attempt fail with the same unhelpful message. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>