Beyond that, avoid using all caps, too many exclamation marks, and hyperbolic phrases ("ACT NOW BEFORE TIME RUNS OUT!!!!"). Poorly formatted HTML in your emails can also hurt how they’re handled. Every spam filter is different, so an email might pass through one filter but get flagged by another. For more comprehensive info on how spam filters work and how to avoid them, check out this guide by MailChimp.
Marketing emails need to be personalized to the reader and filled with interesting graphics. Few people want to read emails that are addressed "Dear Sir/Madam" -- as opposed to their first or last name -- and even fewer people want to read an email that simply gives them a wall of text. Visuals help your recipients quickly understand what the point of the email is.
That isn’t to say that sales-y promotions couldn’t ever create results—as marketing “gurus” have proved. The results just aren’t as good as they could be. Take the biggest gurus’ results—the ones they boast about—and calculate their conversion rates. Often their marketing is comparatively ineffective. They just have massive volume, so the sales numbers are impressive.