Think you know email marketing backwards and forwards? Able to dash off a contact email or sales pitch without a second thought? Check that impulse, because you may get only a fraction of the responses you could receive with some thought and experimentation.

That may not seem surprising if your professional life has been steeped in direct response. Some of the old tested rules of thumb in print would have been startling the first time you heard them, like use the word "you" in copy and write headlines longer than seven or so words. Such fundamentals came from experts who had done extensive testing that pitted one variation against another, time and time again, from many marketers, to learn what patterns tended to hold true.

When it comes to email marketing, mass email vendor MailChimp has just added to what marketers know with a big comparison of subject lines to response rates. The company looked at 24 billion emails sent and 22,000 words in emails sent in the U.S. with tracking. Each email had to go to at least 500 people and the client must have sent at least 10 previous campaigns. In other words, no complete novices and stick to one national demographic for a more reasonable analysis. Even though it was somewhat self-selecting, as only MailChimp clients could wind up in the data pool, the results are still worth considering. Here's what they found.

Personal Works, Chumminess Doesn't

Put the first and last names of recipients into the subject line and you're doing a third of a standard deviation better in open rates than average. That's almost double as effective as using a last name only and close to four times as effective as using a first name only.

And while talking about first names, the effect depends heavily on the industry. Government gets the biggest email open bang for the buck when using a first name only. Creative services and agencies get maybe half that amount. Politics, less. At the bottom is the legal industry, where using someone's first name in the subject line will actually drive many people away from opening the message.

Don't Use the Word 'Free' So Freely

"Free" is supposed to be one of the magic words that make people open messages. When it comes to email subject lines, however, adding free often does nothing. According to MailChimp, on average it helped 0.02 standard deviations. Compare that to the term "freebie," which resulted in a 0.26 standard deviation. In other words, freebie rules. Furthermore, the use of free varies widely in effectiveness, with recruitment and staffing, restaurant, and beauty and personal care enjoying the most benefit. The industries where it did badly, actually lowering response, were surprising--travel and transportation, real estate, and retail saw drops in their response rates.

Keep Things Positive

Creating the perception of time sensitivity is an old direct marketing trick. "Urgent," "breaking," and "important" all performed relatively well. But go too negative, like using the word "cancellation," and you could see a drop in response.

There is more that you should read for yourself. The big point is that the need to test and refine language, images, concepts, and every other aspect of marketing has only become more critical.