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4 Best Practices to Keep Marketing Emails Out of Spam Filters

September 20th, 2022 | 5 min. read

By Kelly Buren

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Spam: It's exactly where you don't want your marketing emails to go. But each year, that's precisely where 18% of these emails wind up here in the United States, according to Return Path's latest report.

If you're sending emails to a list of dedicated clients, it's bad news if 1 in 6 isn't receiving them. Especially when these are the very folks who signed up to hear from you!

Luckily, avoiding the dreaded spam filter is doable with a bit of diligence. At Zoe Marketing & Communications, we've managed email marketing for 15+ years. We send out emails for our own publications, as well as targeted emails for clients. We have a solid grip on what works and what doesn't — and we're mindful of new "triggers" as they emerge.

In this blog, we'll list some of the essential factors to keep your emails "clean" and delivering to your clients' inboxes, including:

  1. Top tips for subject lines

  2. Keeping your images on point

  3. Key tactics for body copy

  4. Other important factors

By the end, you'll feel more confident and less overwhelmed about those big marketing email sends. And you'll have fewer undeliverables and other issues.

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1. Top tips for subject lines

Clear, compelling subject lines are crucial if you want people to open your emails. But certain words, phrases, methods or punctuation can trigger spam filters. 

Also, remember that folks can manually flag your email as spam — and most do it based on reading the subject line. To avoid these fates, follow these tips:

Avoid eye-grabbing gimmicks

If your subject line is shouting at someone, it's rude and annoying. It's also a surefire spam filter magnet. Things to keep in mind:

  • Don't use ALL CAPS. 

  • Don't use multiple exclamation marks (!!!!!).

  • Don't use a mix of upper and lower case — AvOiD tHIs.

Skip the 'infomercial' language

"Limited time!"


"Buy now!"

"Work from home!"

"Earn cash!"

These all have a "carnival barker" feel, and spam filters pick them up fast. Curious about even more "trigger words" to avoid? Browse this list of nearly 400 from HubSpot, which also has breakdowns by industry.

Good practices for subject lines

  • Write in a direct, authentic sentence style. And be succinct. About 40-60 characters are the max — to also ensure it's fully readable on mobile devices.

  • Always check spelling and grammar. Some email platforms do the proofreading for you. You can also pop your writing into a word processor (Microsoft Word, Google Docs) or an AI app like Grammarly for a final sweep.

  • Personalize when you can. Adding someone's first name — a process that can be automated in many email marketing platforms — can boost opens by up to 26%.

  • Use emojis, but sparingly. One is plenty. Research shows they do increase open rates. Plus, they can help shorten up that subject line a bit.

2. Keeping your images on point

Images are a great way to engage people once they've opened your email. But if your art doesn't meet specific specs, it might make spam filters cranky. Here's what to keep in mind.

Don't send only an image — or too many images

A common mistake is putting one designed image file in the body of the email — with all the words embedded into that image — and that's it. It may look pretty, but it throws off your art-to-words ratio. Similarly, too many images can also cause problems. In more detail:

  • Spam filters can't read words on an image, such as a JPG or PNG. So if there's only an image and no text, the filter defaults to the spam folder.

  • Large images or too many images can increase the email's load time and cause the content to get "clipped" or cut off. The filter picks up on this and blocks it.

Other image no-no's

  • Don't include attachments. These can alert spam filters and crank up email size.

  • Don't use Flash, JavaScript or video. They'll make your email far too large and they aren’t widely supported, so likely won’t work correctly anyway.

  • Don't use high-resolution images (300 DPI). These also increase size.

Good practices for email images

  • Use a healthy word-to-art balance. A short headline, anchor image and a few sentences are enough for a single-focus email. For an enewsletter, about 3-10 images are plenty — offsetting each one with short lines of text.

  • JPG and PNG are the most common image files for emails. Make sure they're "web resolution" or 72 DPI. The smaller the file size, the better.

  • Instead of attachments, load any PDFs or other files to your website. Then, link people to that file with a clear call-to-action button.

  • Need some motion? Use a GIF. This is a series of images that instantly plays on a loop. Keep it as short as possible to keep the size down. 200KB is a good max size to aim for.

  • Really want to include a video? Instead, embed a screenshot of your video and link it to the video. Include a little "play button" on the screenshot for added effect.

  • Be sure to keep your entire email under 1MB. This includes all the images, text and code. It prevents your email from getting clipped (when it is, most people stop reading).

3. Key tactics for body copy

This part is critical. If the spam filters get past your subject line, this is the next area they're scrutinizing. Let's take a look at some of the dos and don'ts.

Bad calls with body copy

  • Don't use hyped-up sales language. Also, avoid all-caps and excess bolding, italicizing, punctuation, etc. (Same rules that apply to subject lines.)

  • Don't be erratic with your font size, style and formatting. Having too many headers can be a trigger. Also, "invisible" text (e.g., white text on a white background) is a no-go.

  • Don't do "keyword stuffing." In other words, don't heavily overuse the same specific, targeted terms or phrases. This can be a big red flag for spam.

  • Don't write too much and get "clipped." Again, if folks do get your email, they'll likely stop reading at that point.

Good practices for body copy

  • Keep your writing style simple, direct and conversational.

  • Use consistent fonts that are easy to read. It's OK to use an "H1" (classic header style) for your headline, and, if the body copy is long enough (or for some enewsletter styles), an "H2" (subhead) or two. Otherwise, a simple paragraph font is best.

  • Have a clear call to action or CTA. What do you want people to do? Be clear in your body copy. A CTA button is also a good idea. Long email? Put your CTA higher up.

  • Remember, text balances images. There's no hard-and-fast rule for length, but be tight. Five to 20 words in a headline are ample. For a single-focus email, 25-100 words of description may do it. Some say just 20 lines of text for an enewsletter (200 words) is ideal.

4. Other important factors

Applying general best practices to your emails is a reliable way to avoid spam. Follow these additional guidelines to keep in the filters' good graces:

  • Always include an unsubscribe button. This helps protect your emails' delivery rates. Not to mention, it's required by law. They're usually included in the footer, at the bottom.

  • Never embed a form in your email. It's a security risk, and it's not supported. Instead, link to any form you'd like people to fill with a CTA.

  • Don't buy email lists to "cold email" people. They're seldom clean, which means you'll get bounces. And these folks don't know you or have any affiliation with your brand, so they're more likely to unsubscribe or flag you as spam.

  • For the same reason, occasionally "scrub" your email list. Clean that data and remove bad addresses.

  • Always test your emails before you send them. Email platforms typically have a preview mode. If it's an option, also review how your email displays in different inboxes (Gmail vs. Outlook, for example). 

Next steps for successful email marketing

Landing in a spam filter can cost your business time and money. In this article, you learned some of the best ways to keep your marketing emails safely delivering to inboxes.

The first line of defense is a direct subject line, without goofy gimmicks or "sales speak." Next we dug into images and why keeping file sizes small is essential (along with always including a few lines of text — even if there's text in your image).

We also covered the body copy and how to keep that spam-free. Finally, we dropped several other solid tips, from scrubbing your lists to testing your emails before sending them.

Need a little help launching your targeted marketing email? Let Zoe Marketing & Communications put our 15+ years of marketing email experience to work for you. Click to talk to your advisor.

Or, if you're still exploring, get a broader view of how digital marketing can help you achieve your business goals.

Ready to Reach More Customers?

Connect with your best prospects — and drive more conversions — with a custom mix of digital ads, content, email, streaming ads, SEO, creative services and more.

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Kelly Buren

Beyond serving as Zoe Marketing & Communications’ creative director, Kelly Buren has sharp marketing instincts. Whether designing a magazine or compelling ads for clients, in her 15 years in the field, she’s learned to take risks and test, test, test.