Should you use, or avoid, certain words in your subject lines when sending emails directly to business contacts?

Sidekick, a division of HubSpot, recently examined 6.4 million emails sent by individuals through Gmail, Apple Mail, and Outlook to find out. The emails came from Sidekick's 50,000+ users—including marketers, sales reps, managers, and entrepreneurs—and were all one-to-one messages (i.e., they were not marketing campaigns sent to lists of people).

One-to-one emails with the word "you" in the subject line were opened 5% less compared with those that did not include the word, the analysis found.

Emails with "free" in the subject line were opened 10% more.

Emails with with "quick" in the subject line were opened 17% less.

Emails with "tomorrow" in the subject line were opened 10% more.

Emails with "meeting" in the subject line were opened 7% less.

Emails with "fw:" in the subject line were opened 17% less.

About the research: The report was based on data from an analysis of 6.4 million one-to-one emails sent by individuals through Gmail, Apple Mail, and Outlook.