Currently, marketing emails allow you to add multiple A/B tests, but does not allow multivariate testing. Multivariate testing allows you to learn how multiple variables interact, not just whether one variable change works in isolation.

Multiple A/B tests isn't a replacement because you're not randomising within the same population (so comparing the tests is difficult/impossible), and it'd also be quite time-consuming.

This is particularly valuable for high-volume emails (50k+) where the opportunity for testing is large, and adding only a couple of A/B tests is a missed opportunity. Multivariate testing can give you information that allows you to get the absolute best out of your emails before sending out to the rest of the list.