Email is such a time vampire that it’s tempting to skim through a packed inbox without reading each message too closely, let alone replying to it — and most emails don’t really call for a response anyway. But let’s say you’ve hit “send” on one (or more) that does, and now you’re feeling like a character in Waiting for Godot.

In an attempt to take some of the mystery out of why some messages prompt a reply while others just sit there, email-management service Boomerang recently delved into its database of 40 million users and reported its findings. To improve the odds that a recipient will get back to you, try:

Writing an email that a 9-year-old could easily grasp. Boomerang divvied up messages by reading level—kindergarten, third grade, high school, and college. The third-grade batch, with its short sentences and simple words, won out, with a response rate of 53%. Kindergarten was not far behind (46%), while just 39% of college-reading-level emails got answers.

Adding a little emotion. But not too much. Each answered email in the study got a “sentimentality score” on a scale of 0 (completely neutral, just facts) to 10 (either gushing or ranting). Neutral messages almost never got replies, while the ones that did included some sign of the sender’s mental state, whether positive (friendly and upbeat) or negative (mildly impatient or irritated). As you might expect, “poisonously negative” emails and those containing “excessive flattery” were the most frequently ignored.

Getting to the point. The optimal length is between 50 and 125 words, where response rates topped 50%. Somewhat longer emails, at 150 and even 200 words, did almost as well. Any more verbosity than that, and the odds of a reply drop sharply. “If you need to send War and Peace,” the authors advise, “consider sending it as an attachment.”

Using telegraphic subject lines. The emails that got the most replies had subject lines of only 3 or 4 words, with response rates of 48%. But recipients clearly need some clue, however brief, to what your message is about, since only 14% of emails lacking any subject line got answered.

Asking a question (or three). “We found that emails that asked from one to three questions were 50% more likely to get a response” than those with no questions, says the report. Too many questions, however, are off-putting: “An email with three questions is 20% more likely to get a response than an email with eight or more.”

One more tip, from a different Boomerang study: You’re most likely to snag your recipient’s attention long enough for him or her to shoot off a reply either early in the morning or around lunchtime. Response rates fall off in mid-morning and mid-afternoon.

But suppose you do everything right and still hear nothing. Don’t take it personally, say the folks at Boomerang: As email marketers know from long experience, “even the most-optimized emails won’t get a 100% response rate.”