If you want to get a reply to your email, you can start by writing a good one. Boomerang has had some helpful tips to do this in the past, but now the company is taking it a step further. Its new feature Respondable will rate your emails on the fly in a few key categories. Respondable works by displaying a series of recommendation meters for things like subject length, word count, reading level and positivity (though you'll have to be a Pro user for some advanced features like positivity). If you start to write a really long email, use too much jargon or are too rude, Respondable will let you know. Obviously, you don't have to adhere strictly to Respondable's suggestions, but they're handy to have.
The new feature is rolling out with a new version of Boomerang. If you're currently using Boomerang in Gmail, Google Apps, Microsoft Office 365 or Outlook, you should see it in the latest update.