The research also found the click-to-open rates increase as subject lines climb to over 100 characters.
That's important for marketers, since click-to-open rates gauge whether consumers opening an email also click-through to possibly make a transaction or pursue further information.
The 70-words-plus thesis is based on the concept that a longer subject line permits a more detailed description of "what's in the email and by corollary, more of what's not," said Alchemy Worx CEO Dela Quist. "The more information you can put in a subject line, the more likely you are to send relevant people to the proposition," Quist said.
On suggestions that shorter subject lines are more effective, Quist said subject lines that are 50 characters or less (or fewer than six words) do generate high open rates, but not a high number of click throughs.
The shorter subject lines are "more likely to be ambiguous or misleading," he noted. Spammers generate high open rates from the shorter subject lines, due to their ambiguity, but low click-through levels.
Quist said there is an exception to the overriding conclusions -- when a short subject line can yield a high click-to-open rate. That's if there is a powerful single message such as "Nike trainers -- 50% off" that conveys the email contents in a concise way to relevant consumers. "That's a subject line you can't improve by making it longer," he said.
The Alchemy Worx conclusions are based on an analysis of more than 600 subject lines and 200 million email messages. Quist also said the research found there's a "dead zone," where subject lines between 60 and 70 characters (six to 10 words) fail to optimize either the open rates or click-to-open rates.
Alchemy Worx works on email marketing campaigns for clients such as PayPal, Hilton and Deloitte.