"We recommend testing only one difference between the A and B groups. When there are several differences between test groups, it’s difficult to figure out which change impacted your engagement."

A/B testing may be better to figure out one specific aspect (time to send, sender name) but wouldn't multivariate testing make sense if you were looking for the best combination of those variables? i.e. if I wanted to find out that sending an email at 9pm, from a corporate account, with larger images is the best combination?

I'm interested to hear others' opinions on the pros/cons.