The Deliverability Advantages of Custom HTML Email Designs

If you ask most marketers what affects email deliverability, they’ll point straight to email authentication, infrastructure, and sender reputation. SPF, DKIM, DMARC—those three letters get tossed around like gospel. And yes, they matter. So much that many teams treat them as the whole story.

That mindset pushes design into a separate, “nice-to-have” bucket. Something you worry about for engagement or branding, but not for inbox placement. 

The twist: while it would be a stretch to say email design has a direct line to deliverability, it undoubtedly influences the signals that inbox providers watch. 

Clean, fast-loading, and smartly formatted layouts of custom HTML email designs mint higher opens and clicks. And those engagement metrics are the signals Gmail and Yahoo watch to decide your sender reputation. 

That’s why we’re zooming in on custom HTML email design. Not as the magic lamp, but as a tiny but crucial screw in the bigger machine of deliverability. 

In this blog, we’ll unpack how email designs shape inbox placement—and the practices that give your campaigns a deliverability boost. 

What Email Deliverability Really Means

When marketers talk about the deliverability of their custom HTML email templates, they’re not just asking, “Did my email get sent?”That’s email delivery. 

Email deliverability is the percentage of emails that reach the subscriber’s inbox without getting filtered into spam, junk, or other folders.

Said another way, it’s not if your email left the server, it’s where it landed. 

Because an email may technically be delivered to a server, but if Gmail parks it in Promotions or spam, it’s as good as invisible. Similar to the difference between a package arriving at your door and whether you bring it inside or leave it on the porch.

For years, inbox placement felt like a black box. As email strategist Chad White put it in this Mavlers video, experts were “reading the tea leaves” to guess what Gmail, Yahoo, or Microsoft wanted. 

That changed when the big players started spelling out requirements—DMARC authentication, one-click unsubscribe, and stricter spam-complaint thresholds among them. Thankfully, now it’s less guesswork, more clarity.

But identity checks and infrastructure only take you so far. “Inbox providers ultimately reward engagement,” White emphasizes. Opens, clicks, and interactions tell ISPs whether subscribers value your messages. The levers are simple but not easy:

• Send smarter. Personalize, segment, and automate. That nails the timing of your email sends.
• Send less. Pull back from inactive subscribers before they mess up with your engagement metrics.

In short, email deliverability is about proving you deserve your spot in the inbox. 

Finally, Designmodo’s “periodic table for deliverability” creatively sums up the factors that influence inbox placement and makes a handy reference to keep nearby.

Image Source: Designmodo

The Deliverability Risks of Bad Email Design

It’s comforting to believe that mailbox providers exist just to punish senders—because that shifts the blame away from marketers. But that’s not the reality. Their real objective is to protect end users, keeping inboxes safe from phishing, scams, and poor experiences.

But the way your custom HTML email design is built can either help you pass spam filters or land you squarely in spam. 

And know that the spam filters aren’t just parsing subject lines. They scan formatting, image-to-text ratios, colors, and even code bloat to size up your email for its authenticity. 

As Drupal Sinh Barad, project manager at Mavlers, explains:

“Deliverability is like trying to get into an exclusive club—your email needs the right ‘dress code’ to pass the ISP bouncers. DIY templates often generate bloated code with redundant elements, while overly complex custom designs can cause rendering issues. Both can hurt inbox placement if not optimized.”

So what does “bad design” really look like in the eyes of a filter or a frustrated user who is about to send your email to the spam folder?

• Spam filter triggers. Overly image-laden layouts, suspicious fonts, or aggressive color palettes often mimic phishing attempts. The result is blocked emails or a straight trip to spam.
• Subscriber behavior. If a reader opens your email and immediately marks it as spam because of offensive or misleading images, you’re waving a red flag sending to inbox providers.
• Image-heavy emails. Many email clients don’t automatically display images when an email is opened. If your design leans too hard on visuals with little to no supporting text, there is an inherent risk of disengagement and complaints.
• Scroll fatigue. Long, overloaded email layouts mean endless scrolling, especially painful on mobile. Most readers will abandon it within seconds, affecting your engagement metrics.
• Slow or broken visuals. Large image files that stall or pixelate hamper user experience. It’s not the slow load alone that wrecks deliverability, it’s how users react (deletes, unsubscribes, complaints).
• Phishing attempts. Deceptive imagery or CTAs are the fastest way to land in the spam folder. Filters are ruthless here, and rightfully so.
• Gmail clipping. Emails over ~102KB get clipped. Worse, if your unsubscribe link lives at the bottom, a clipped email makes it harder for people to leave gracefully. Then, they do the one thing that hurts most: hit “Report Spam.”
• Other risky tactics. URL shorteners, repetitive subject lines, or sending from free domains may feel convenient but hurt trust instantly.

The Deliverability Best Practices For Custom HTML Email Design

Good custom HTML email layout design amplifies deliverability because it minimizes friction. This, in turn, sends the right signals to inbox providers and makes your emails easy to love (and hard to mark as spam). 

Here are the essentials every sender needs to get right about their custom HTML email designs: 

1. Make it responsive
   71.5 % of customers prefer mobile devices when it comes to email checking rituals. Needless to say, any email that goes out should inevitably adjust automatically across screen size variations.

Nonetheless, a single-column layout is a failsafe option if you’re unsure about how your email copy renders on smaller screens. 

2. Keep images smart

Images were a major part of the overall motivation behind the shift from plain-text to HTML emails.  But that doesn’t mean they can do whatever they want without hurting your deliverability.

• Compress images to keep slow load times in check.
• Use images in the correct and relevant format. JPEG for photos, PNG for graphics, GIF for simple animations.
• Size them beforehand to avoid distortion.
• Make writing descriptive ALT text to image part of your muscle memory.
• Don’t let images be the message. Use HTML text for key content.

3. Preserve fonts and readability
   Set the body text to the 14–16px range, headings to 22px or above. Prefer web-safe fonts. Fonts like Arial or Verdana favours consistent rendering. If custom fonts are the only way you want it to be,  back it up with good fallback fonts.
4. Don’t overcomplicate CTAs.
   Use text-based CTAs, not just image buttons, since blocked images = invisible CTAs. Make them large enough for thumbs, leave space between links, and give them contrast so they emerge without being spammy.
5. Use white space and hierarchy.
   A clean design with breathing spaces is where your email message thrives. It becomes easier to scan and spot your CTA. If you prioritize only the most valuable information up top, the layout becomes much more user-friendly. Breaking the text wall into readable chunks and sticking to consistent branding elements also greatly helps in easing the overall layout.
6. Refrain from spammy signals

• Skip the funky fonts and “FREE!!!” subject lines , unless they truly fit the message.
• Don’t use URL shorteners as filters, and subscribers both see them as shady.
• Keep total email size under 102KB so Gmail doesn’t clip your footer (and your unsubscribe link along with it).

7. Respect the unsubscribe.
   The readers who don’t want your email? Escort them out like a gentleman. Always have an easy-to-spot unsubscribe link in each of your custom HTML email designs.

It saves you from spam complaints—the #1 factor dragging down deliverability. In fact, 54% of users report spam when they didn’t give permission, and 49% do it if they can’t find the unsubscribe option.

8. Stay relevant.
   Every image, every line of copy should earn its place. Random visuals dilute your message. Consistent, purposeful design, on the other hand, builds recognition and trust over time.

Or as Drupal Sinh Barad, project manager at Mavlers, puts it:
“DIY templates often come with bloated code, while overengineered custom designs risk rendering issues. The key is balance—clean, responsive HTML that loads fast, reads well, and works everywhere.”

Wrapping Up

Optimizing your custom HTML email design for deliverability is complex, with tons of ins and outs. The question isn’t just whether to adapt layouts yourself or rely on a prebuilt template. It’s about knowing what design elements impact inbox placement and how. 

For teams sending high volumes or running intricate workflows, working with a custom email design service ensures your email templates support deliverability. 

Smaller teams can still benefit from applying these best practices thoughtfully.