This is one of the foundational blocks of good writing. Active voice has multiple benefits: it's easier to understand, it usually requires fewer words, and it makes a message stronger and more exciting. Compare "More than 10,000 customers have been helped by our service" to "We have helped more than 10,000 customers."
In a sales email, passive voice makes you sound weak and unconvincing, which won't help you build trust with a potential customer. There's a time and place for passive voice in writing, but you'll pretty much never encounter it while doing sales emails, so it's best to follow the rule of avoiding it altogether.