What happens when you go over your plan
Leads over the monthly allowance are still stored and held; delivery waits.
The monthly lead allowance never makes a lead disappear.
Stored first
When a lead arrives the plan allowance is not consulted. The lead is stored first, within your plan's storage limit. Only afterwards is it counted.
Delivery waits
A lead past the allowance is marked as over plan, held, and not delivered until your allowance resets or you upgrade. You get a notice. The pricing page shows the allowance of each plan.
Spam is not the same thing
Spam never counts toward your allowance. A lead held as possible spam goes to quarantine and is kept for 30 days.