A/B testing can be used to optimize email marketing campaigns by testing different subject lines, content, call-to-actions, and offer types to see which perform best for key metrics like open and click-through rates. The document recommends testing different offer types, age groups, and geographies to better understand what resonates most with different audience segments. It also suggests making emails stand out in inboxes, giving subscribers control, easing them in with a welcome series, understanding why people unsubscribe, and analyzing campaign performance by acquisition source.