Vendor rate limit from shared API key

Your transactional email pipeline (password resets, receipts) starts lagging at 11:00, emails that should send in seconds now take 20+ minutes, and some fail. Dashboards: your email vendor (SendGrid-style) API returns a rising share of HTTP 429 'rate limit exceeded' with Retry-After headers; your send queue depth is climbing steadily; your app is otherwise healthy. Recent context: at 10:45 a one-off marketing batch job started blasting a 2M-recipient newsletter through the same API key / account as your transactional mail. How do you triage and mitigate?