Ten lookups, and the eleventh fails everything
The sender-policy record grew one include at a time — the CRM, the helpdesk, the newsletter, the payroll vendor, the analytics tool somebody added last quarter. Each addition was one line, and nobody was counting.
Then a sender that had worked for two years started failing, and nothing about that sender changed. It is simply the last name in a list that is only allowed to cost ten lookups. Going over does not fail that one sender — it returns a permanent error for the whole evaluation.
Outside every boundary: The receiving server (reads it left to right), Your sender record (one line at a time), The helpdesk include (four, three nested), The CRM include (one lookup), The payroll vendor (lookup number eleven; FAILED: permerror), an empty slot for the spends none of the ten Connections: Your sender record calls The CRM include (step 1) Your sender record calls The helpdesk include — and three inside it (step 2) Your sender record calls The payroll vendor — lookup eleven (step 3) The payroll vendor calls The receiving server — permerror (step 4) spends none of the ten controls Your sender record