For the past three years, I've been writing a free weekly newsletter where I critique and rewrite my readers' sales messages. I've also been working with companies to test out different sales emails to see what's most effective.

As part of this process, I've encountered research and advice from email marketing companies about how to get your sales emails opened. Unfortunately, some of the advice that's floating around on the Web is highly misleading, IMHO. Let me explain.

Obviously, the Subject line of an email greatly influences whether or not that email is opened. The most easily quantifiable part of a Subject line is its length, and that's where the confusion sets in.

Some experts believe (wrongly in my view) that the ideal length for a Subject line is around six to 10 words. For example, the online marketing company WebpageFX recently compiled the average number of words in Subject lines from 6,713 email newsletters sent by 146 online retailers over the holiday season.

Based on those averages, the company recommends "keeping your subject lines between 6-9 words and 36-50 characters."

While that seems like "scientific" advice, the methodology for the WebpageFX study doesn't hold water. Calculating the average lengths of the Subject lines of those emails only tells you is what those companies did, not how well it worked!

When I pointed this out to WebpageFX, they stated that their findings and recommendations were compatible with a study by Madhu Gulati of ShowMeLeads, a company that consults for customers of Marketo's email marketing engine.

Gulati claims that, according to a study of 260 million emails sent as part of 540 email campaigns (presumably using the Marketo engine), Subject word lengths had the following effect on open rates, in descending order of effectiveness:

  • Six to 10 words: 21 percent open rate
  • Five words or fewer: 16 percent open rate
  • Eleven words or more: 14 percent open rate

Again, that seems definitive. However, Gulati immediately calls her own findings into question by stating that "succinctness is useful to attract customers." If that were true, why would five words be less effective than 10 words?

Another potential problem with Gulati's data is that it may include data from before mobile devices began to dominate email usage. Today, 65 percent of all emails are opened on mobile devices. Since the inboxes on mobile devices are typically displayed in portrait mode, Subject lines longer than five words will probably get truncated.

A better set of recommendations (IMHO) comes from the email marketing firm ContactMonkey, which conducted a study of 30 million Gmail and Outlook emails.

According to that study, Subject lines with three words were, on average, only opened 15 percent of the time, with that percentage declining as more words are added. The study also revealed that the Subject lines most frequently opened were:

  1. "Re:" (92 percent open rate)
  2. "Re: Follow Up" (90 percent open rate)
  3. "Re: Update" (89 percent open rate)
  4. "Re: Introduction" (88 percent open rate)
  5. "Re: Checking" (87 percent open rate)

Since those open rates are so much larger than Gulati's study of marketing emails, it's fair to assume that ContactMonkey was tracking both internal and external emails. (As a general rule, emails sent from inside the same company are opened more frequently than those from outside.)

However, my own experience with my free weekly newsletter strongly suggests that the ContactMonkey study accurately represents the optimal length for Subject lines for outside (i.e. sales) emails. Here are the Subject lines and open rates for my eight most recent newsletters, in descending order of effectiveness:

  1. Re: Steve Jobs (25 percent)
  2. Re: Executive Summaries (24 percent)
  3. Cold-Calling Secrets (21 percent)
  4. Workplace Happiness (20 percent)
  5. Email Marketing Secrets (19 percent)
  6. The 10 Commandments of Leadership (19 percent)
  7. How to Land Your Dream Job (18 percent)
  8. Get 25x More Qualified Sales Leads (17 percent)

As you can see, my experience tracks exactly as the ContactMonkey study predicted, including the effect of the "Re:" prefix.

My experience that short Subject lines are more effective has been reinforced since I've worked with several companies on improving the performance of their sales emails. While reducing the number of words in the Subject line is only one of my recommendations, I have clients who are getting open rates exceeding 70 percent for their sales emails.

So here's the takeaway. If you want your emails to be opened:

  1. Shorten your email subject lines to two words, three maximum.
  2. If appropriate, use the prefix "Re:"
  3. Ignore all advice to the contrary.

In future posts, I'll describe how to choose words for your Subject lines that will further increase the percentage of your emails that get opened.