Emails with the word "alert" in their subject lines have a 38.1% higher than average open rate and 61.8% higher click rate, according to a recent study by British marketing firm Adestra.

The keywords "free delivery" (+50.7% higher open rate, +135.4% click rate) and "bulletin" (+15.8%, +12.7%) also performed very well in the email campaigns analyzed.

On the other hand, "report" (-23.7% average lower open rate, -54.8% click rate), "learn" (-35.5%, -60.8%), and "book" (-4.6%, -25.4%) had a negative effect. "Newsletter" showed a marginal effect on open rates (+0.7%), but had an adverse effect on click rates (-18.7%.)

As for date-related keywords, "daily" (+27.8%, +100.3%) and "weekly" (+27.1%, +50.6%) performed strongly, but "monthly" (-26.6%, -37.0%) had a negative effect.

Below, additional email subject line keyword performance broken out by B2B, B2C and commerce. For complete results and analysis check out the full study, The 2013 Adestra Subject Line Analysis Report, which was based on a review of over 2 billion global emails.

B2B Emails

The words "alert" and "breaking" in the email subject lines of B2B emails performed well.

B2B customers seem to have become desensitized to words such as "reports," "forecasts," and "intelligence."

B2C Emails

"Review," "update," and "special" all did well in the subject lines of B2C emails, as did "video."

The use of question marks in B2C subject lines had a negative effect.

Retail and Commerce Email

"Free delivery" (+35.9% higher than average open rate, +81.3% higher click rate) performed very well in retail and commerce email subject lines.

Consumers love a "sale" (+10.7%, +26.7%) and specific offers such as percent off, (+6.1%, +17.7%).

Generic offers such as "save" (-4.4%, -27.4%) and calls to action such as "buy" (-19.3%, -59.1%) had a negative effect.

"Cheap" (-67.2%, -71.6%) and "free" (-23.7%, -34.8%) also resulted in lower than average performance.

About the research: The study was based on an analysis of a random sample of over 90,000 email campaigns, each with a list size of at least 5,000 subscribers, for a total of over 2 billion emails.