Content and design factors: subject lines, html, images, links, spam triggers

Content is not the primary driver of deliverability anymore, but it still matters. Mailbox providers weigh reputation and engagement much more heavily than the words in your subject line or the number of images in your email. That said, you cannot ignore content. If your messages look or feel like spam, they are far more likely to get filtered.

Subject lines
Subject lines are often the first filter. Overly aggressive language like “Free!!!” or “Act now” can trigger suspicion. Subject lines that mislead or overpromise also hurt engagement, which indirectly damages your reputation. Aim for clarity and relevance, not tricks.

HTML quality
Clean code helps. Emails with broken tags, sloppy formatting, or bloated templates may appear untrustworthy. Most ESPs provide tested templates, but if you code your own, make sure it is lightweight and valid.

Images and text balance
Emails that are just one big image or have very little text can look suspicious. A balanced ratio of text to images is a safer bet. Always use alt text for accessibility and to give mailbox providers something to read.

Links
Too many links, mismatched display and destination URLs, or linking to sketchy domains can set off filters. Keep links relevant and consistent with your brand domain.

Spam triggers
While keyword filtering is less powerful today, certain phrases and patterns can still hurt. Excessive punctuation, all caps, or messages packed with “free,” “limited time,” and similar phrases can be red flags.

The bigger picture
Content and design are not the top factor in deliverability anymore. ISPs rely much more on your sender reputation and how subscribers interact with your emails. Still, sending spammy-looking messages will work against you. Clean, clear, and honest emails create trust and support the stronger signals of engagement and reputation.

In article 6, we will look at infrastructure and sending practices, and how setup choices like shared versus dedicated IPs and proper warming can influence inbox placement.

1. Intro to email deliverability

2. Authentication 101: SPF, DKIM, DMARC explained simply

3. How ISPs judge your emails: reputation, engagement, spam traps, complaints

4. List hygiene and data quality: why clean lists matter more than big lists

5. Content and design factors: subject lines, html, images, links, spam triggers

6. Infrastructure and sending practices: shared vs dedicated IP, warming, throttling

7. Monitoring and troubleshooting: how to use seed tests, blocklist checks, analytics

8. Future of deliverability: AI filters, gmail and yahoo changes, privacy trends

Next article →

This post is part of our deliverability series. Check out the full guide here.