How do mailbox providers test experimental filters?

Mailbox providers test experimental filters through controlled rollouts that apply new filtering to small traffic samples before broader deployment. A new model might filter 1% of incoming mail while engineers monitor for unintended consequences.

A/B testing compares new filter performance against existing systems. Metrics include spam catch rate, false positive rate, user complaints about missed spam or wrongly filtered mail, and overall user engagement. New filters must improve metrics without regression.

Shadow mode runs new filters in parallel with production systems, logging decisions without enforcing them. This reveals how the new filter would behave at scale without risking user experience. Only after extensive shadow testing do filters graduate to production enforcement.