In the beginning, marketing automation platforms basically involved if/then rules: if a customer takes this action, show this response. But overlapping e-mail campaigns with if/then rules become very complicated very quickly, especially when you're talking about millions of customers, each one in a different frame of mind, and each expecting his/her own personalized experience. As a result, marketing platforms are evolving to the newer approach of customer journeys that are often guided by machine learning.
