Just as discretion is the greater part of valor, editing is the greater part of copywriting, particularly for email. The long vs. short copy debate has waged for years, predating email by decades. I’ve defended both camps at different points in my career, and for different applications. Traditionally, the answer to the question of what copy length is best vacillates between “it depends” and “the right length is as long as you can go and still hold their attention.”

But in 2010 there are some new factors weighing pretty heavily on the copy length conundrum, that have less to do with direct marketing and more to do with technology adoption and trends. I believe that for promotional messages, short form copy is going rise in importance, and can better support email marketers with some of their near-term and long-term objectives. Here’s why:

1. Attention is at an all time premium. It’s continuing to get harder to flag someone down – even your best customers who look forward to hearing from you. Inboxes are crowded; mobile devices don’t invite the same immersive reading environment; marketing messages are showing up everywhere, from stairways to elevators and even on the foreheads of people on these stairways and elevators. Short, sharp email messages can poke through the clutter.

2. Social media is compressing the window of comprehension. Ask someone who spends a lot of time on Facebook if they start viewing the world as a possible status update. Or inquire if any of your Twitter friends think in 140 characters or less. Social media has many people accustomed to communicating in staccato-like machine gun bursts, never mind full paragraphs or even complete sentences. I’m not saying your emails should abandon grammar or structure. But continuous exposure to 20 word communications can make your 500 word email seem impossibly burdensome to some of your subscribers.

3. Engagement metrics will place a high value on singular-purpose messaging. One takeaway I got from my interview with Dean Canellos on engagement metrics is that sending emails that subscribers simply ignore will begin to have deliverability consequences. As engagement rises as an email marketing objective, senders should begin experimenting with different types of messages designed to drive a click-through or other activity. Messages with a singular focused call to action may get the results that previous efforts didn’t deliver. Focus on driving a single click-through and let the landing page do more of the content merchandising work.

4. Increased relevance means more message versions. Another take away from the Dean interview is the need for more list segmentation so that better targeted messages can reach each subscriber. Alternate versions will lead to more work, and more work is easier to achieve when the messages are shorter. So not only are shorter messages a strategic trend; they’re a practical consideration. You could wind up with better results, greater engagement, increase segmentation and multiple creative versions, at little or no increase in workload.

There is still a healthy dose of “it depends” in all of these recommendations, however. And the beauty of email marketing is that the rich metrics all you to challenge and test each one of them against your own benchmarks and objectives. But the macro trends in place are powerful and it’s our job as marketers to anticipate and respond to the shifting landscape.

It’s also the job of copywriters – particularly those who charge by the word. If that’s you, I’d recommend exploring a new pricing model, just in case I’m right.