How can you increase the likelihood your emails receive responses? Step one: Dumb them down. That's one lesson Boomerang CEO Alex Moore drew from the machine learning technology behind a new feature of his company's email plug-in.

The Mountain View, California, software startup's new feature, called Respondable, tells users how likely their email is to get a reply by scoring characteristics of its content such as length and tone. It's one of several available tools aimed at augmenting and automating email functions. Another is the website Crystal, which claims to divine how you should address specific individuals through online communication.

Between the two apps--Boomerang also offers features like scheduled email and automatic resending--there's a lot of advice on how to write an effective email. Here are six of their best tips.

1. Keep the language simple

Pick the two-syllable word over the five-syllable word, the short sentence with simple syntax over one that's cut in half by a semicolon. Basically, aim for a third-grade reading level. "It makes a huge difference" in whether the email receives a response, Moore says. "I've been really trying to pay attention and simplify my language."

2. Politeness may not matter that much

Moore says algorithms used in Respondable indicate politeness has the lowest correlation with outcome. He adds, however, that the correlation may be low because it's based on email data, which tends to trend polite anyway. "We don't send that many emails that are just plain rude," he says. The point is: You're probably as polite naturally as you need to be.

3. Don't be neutral

If you can't say something nice, say something negative. You probably want to steer clear of the middle ground. Moore admits the advice sounds counterintuitive, given that it's based on machine-learning algorithms, but says it's also comforting. Even with artificial intelligence technology guiding your writing, "We're still dealing with humans. People want to respond to emotion." That's not to say Respondable approves of just any negative content. Profanity, for instance, doesn't help: The app docked the score of one email I wrote to a friend because I used the term "bullshit."

4. Take an individual approach

People have different communication styles, says Crystal head of growth Luke Thomas. "Based on that premise, the most important thing you can do is recognize that your communication style is not the gold standard." Ascertain what you can about how a person communicates. Does she tend to respond better to short, declarative sentences? Does he enjoy argument and debate? Should you speak bluntly, or with a little more subtlety? "You should try to meet in the middle and adapt," Thomas says.

5. Say what you need to say

It can be tempting to try to follow every bit of machine-tested advice to a tee. You might want to squeeze in a number of phrases Crystal suggests a person would like to hear, or craft an email that pushes Respondable's green score bar further along as you type. But you're better off just using these tools to tweak your messages rather than to guide your writing from start to finish. "Say what you need to say and see if it works," says Moore. "A lot of the time it does."