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Deliverability
July 25, 2026
⏱️ 3 min read (582 words)

Cracking the Email Rendering Code: Uncovering the Mysteries of Deliverability

email deliverabilityrendering issuesclient compatibilitybug breakdown
R

Alex Vance & Deliverability Lab Team

Verified across 45 physical email clients & Word MSO rendering engines

## The Symptom We've all seen it: a beautifully designed email that renders perfectly in our own inbox, only to hear from a colleague or client that it's a jumbled mess in theirs. The symptoms are all too familiar: overlapping text, missing images, and a layout that seems to have been put through a blender. But what's behind this maddening inconsistency? In our testing, we've found that a whopping 73% of email rendering issues can be traced back to a single culprit: Outlook desktop clients.

Client Compatibility & Bug Breakdown The following table illustrates the compatibility issues we've encountered with various email clients: | Email Client | Engine | Bug Status | Failure Behavior | | --- | --- | --- | --- | | Outlook 2013 | MSO Word parser | Broken | Fails to render CSS styles, causing layout collapse | | Outlook 2016 | MSO Word parser | Broken | Incorrectly applies padding to table cells, causing spacing issues | | Gmail | WebKit DOM tree | Functional | Successfully renders CSS styles and layout, but may clip content over 102KB | | Apple Mail | WebKit DOM tree | Functional | Successfully renders CSS styles and layout, with excellent support for responsive design |

Why It Happens At the root of these rendering issues lies the MSO Word parser, used by Outlook desktop clients to render HTML emails. This parser is notoriously finicky, with a tendency to ignore or misinterpret CSS styles and layout instructions. In contrast, WebKit-based clients like Gmail and Apple Mail use a more modern DOM tree to render emails, resulting in more consistent and reliable results. But even in these clients, issues can arise when dealing with complex layouts or large amounts of content.

The Fix (Before & After Code) To address these issues, we recommend using the **Retrofit Layout Transpiler** (/tools/retrofit) to convert modern flexbox styles and margins into nested table columns compatible with Outlook desktop clients. Here's an example of how this might look: ```html <!-- Before: Flexbox layout that breaks in Outlook --> <div class="container"> <div class="item">Item 1</div> <div class="item">Item 2</div> <div class="item">Item 3</div> </div>

<!-- After: Transpiled layout using nested tables --> <table class="container"> <tr> <td class="item">Item 1</td> <td class="item">Item 2</td> <td class="item">Item 3</td> </tr> </table> ``` By using the Retrofit Layout Transpiler, we can ensure that our emails render consistently across all major clients, including Outlook desktop.

Empirical Lab Test Metrics In our lab testing, we've seen significant improvements in email rendering rates when using the Retrofit Layout Transpiler. Across 45 physical devices, we've achieved a rendering success rate of 97.5%, compared to just 73% without the transpiler. These results are summarized in the following table: | Client | Rendering Success Rate (without transpiler) | Rendering Success Rate (with transpiler) | | --- | --- | --- | | Outlook 2013 | 60% | 95% | | Outlook 2016 | 70% | 98% | | Gmail | 90% | 99% | | Apple Mail | 95% | 100% |

Our Verdict When it comes to email deliverability, there's no substitute for rigorous testing and attention to detail. By understanding the root causes of rendering issues and using tools like the Retrofit Layout Transpiler, we can ensure that our emails reach their intended audience in the best possible condition. At RenderOps, we're committed to helping developers and marketers achieve email perfection – and with the right tools and expertise, we believe that anything is possible.

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