Before emailing someone busy and important, break out your phone for a final test

If you're emailing someone who's even remotely busy, there's one way to ensure that your message gets ignored: Make it long. Scary long.

They'll scan it and say they'll come back to it later and then, in all likelihood, they never will.

The question is: How do you know if you've gone overboard? According to Jocelyn K. Glei, author of "Unsubscribe: How to Kill Email Anxiety, Avoid Distractions, and Get Real Work Done," all you've got to do is preview the email on your phone and see whether it looks too lengthy.

It's that simple.

After all, one recent survey found that more than half of emails are opened for the first time on a mobile device.

In an interview with Business Insider, Glei said, "Sometimes something you wrote on a desktop that looks reasonable, when you put it on an iPhone, it looks like 'War and Peace.' So then you want to go back and ruthlessly edit."

She added: "You always have to take into account that someone will be processing that message on-the-go, in an impatient state, at a glance." That means you need to "be concise and to get right to the point."

If for some reason you can't preview the email on your phone, Glei said you can simply size the email window on your computer down to the phone size.

And while you're at it, be thoughtful about your opening line. Here's Glei:

"Think about that two-line message preview that someone's going to see when they're scrolling through their inbox on their iPhone. Is that going to get their attention, even just to open the email in the first place?

"I think we have to think that strategically at this point, because at this point everyone is overwhelmed with email and is just so busy in general."