HTML Does Not Hurt Deliverability. Your Sender Reputation Does.
HTML Does Not Hurt Deliverability. Your Sender Reputation Does.
I have seen brands switch to plain text emails because they read that HTML hurts deliverability. Their inbox rates did not improve. Their brand experience dropped. The myth cost them something real, and it was built on a misreading of what mailbox providers actually measure.
What Is Actually Happening
The claim that HTML hurts deliverability has been circulating since the early days of spam filtering and it has not become more accurate with age. In 2026, it is still repeated as deliverability advice in forums, blog posts, and cold outreach playbooks. It is a partial truth, distorted so far from its origin that it functions as misinformation.
Here is where the idea came from. In the early 2000s, when spam was primarily a volume problem and content-based filtering was the dominant approach, tools like SpamAssassin assigned scores to email content. Certain HTML patterns, particularly image-only emails with no text content, were correlated with spam. Spammers at the time frequently used single-image HTML emails to bypass keyword-based text filters. Image-only format became a spam signal.
The correlation was real. The causation was wrong. The format did not cause the spam problem. The format happened to be used by people who were running spam campaigns. An authenticated sender with a healthy list sending a well-coded HTML email to subscribers who wanted it was not, and is not, penalised for the format.
Modern mailbox providers at Google and Yahoo evaluate sender reputation and engagement as the primary inbox placement signals, per Google's Sender Guidelines 2025 and Yahoo's Sender Requirements 2025. Authentication matters: SPF, DKIM, and DMARC are non-negotiable in 2026. Engagement matters: if a significant portion of your list is not opening, clicking, or is actively marking your mail as spam, your reputation suffers. List hygiene matters. Domain and IP reputation matter.
HTML as a format is not in that list. The markup is not what Gmail's algorithms are measuring when they decide where your email lands. In every deliverability analysis I have run, format type has not been the variable that explains inbox placement differences. The variables that matter are on the sender side, not the template side.
Where HTML can cause a deliverability problem is through broken implementation. Excessively large HTML files can trigger issues in some environments. Poor code that produces enormous message sizes, or certain deprecated HTML patterns, can occasionally flag. But that is a code quality problem, not an HTML problem. A well-built HTML email from an authenticated sender with good engagement lands in the inbox.
I have tested this directly: the same sender, the same list, the same authentication setup, with a plain text send and an HTML send within the same week. Inbox placement rates are comparable. The format does not move the needle when the sender is healthy.
When the sender is unhealthy, switching to plain text does not fix the underlying problem. It just gives you a worse-looking email with the same deliverability issues.
The One Fix
If you are worried about deliverability, audit the actual deliverability variables. Do not audit your template format.
Start with authentication. Is your SPF record set? Is DKIM configured and aligned? Do you have a DMARC record published? These three are the foundation. Google and Yahoo's 2025 bulk sender requirements made them non-optional. If any of them are missing or misconfigured, that is your deliverability problem. A plain text email from an unauthenticated domain will land in spam faster than an HTML email from an authenticated one.
Then look at list health. What percentage of your list has not engaged in the last 60 to 90 days? What is your spam complaint rate? These are the engagement signals that mailbox providers use to build your sender reputation. A disengaged list sending to unengaged subscribers is a reputation problem. It does not matter whether the emails are HTML or plain text.
If both authentication and list health are clean, check your HTML code quality as a secondary step. Use render testing tools like Litmus, Email on Acid, or Testi@ to validate your email renders correctly across clients. A well-coded HTML email should have no deliverability issues originating from its format.
What Good Looks Like
When deliverability is working, it is not because of format choices. It is because the sender has clean authentication, a healthy engaged list, and a consistent sending pattern that builds domain reputation over time. HTML emails from that sender land in the inbox alongside plain text emails from the same sender, at comparable rates.
Treating format as the deliverability variable is a distraction from the variables that actually matter. Every week spent worrying about HTML is a week not spent on list hygiene, engagement re-activation, or authentication auditing. Those are the levers that move inbox placement.
The HTML fear also has a real cost. Plain text email loses the branded experience, the product imagery, the design language you have built. It trades a capability you actually have for an advantage that the evidence does not support.
That is a bad trade, and it is made repeatedly by brands that absorbed a myth without tracing it back to its source.