Have ever created a marketing email that appeared flawless in your editor but was disrupted in the inbox of your subscriber? One of the most significant issues for marketers and designers is rendering accuracy in 2025. Despite the fact that dozens of smart tools can be used, all email clients, Gmail, Outlook, and Apple Mail, still interpret HTML differently.
What was a perfect image in one application can become a nightmare in the other. That is why rendering precision is not a feature to be desired; it is needed. Research indicates that two-thirds of users remove emails that have broken or ill-formatted email messages. In the case of brands, that translates to lost contact and reduced conversions. Selecting the appropriate HTML email builder today is not only a time-saving option but also a protection of the uniformity of the brand in all inboxes.
The Builders Compared
We compared the results of four popular HTML email builders to determine which of them provides reliable rendering: Stripo, BeeFree, MJML, and Unlayer. Each of them was rated in terms of usability, quality of code, and compatibility with the key clients. Stripo was impressive in its drag-and-drop building and always produced clean HTML output. BeeFree was fast and easy, but it had to have small CSS adjustments to work with Outlook. MJML is a developer-friendly framework that generates responsive templates that work well on all devices, but requires some technical installation.
Unlayer was also unique in teamwork, but failed at times with the older rendering engine of Outlook. Based on actual tests, Stripo was the most balanced experience that beginners could easily use, but precise enough to satisfy the professionals. MJML was right behind, and it is ideal when a person needs full control over his/her code. It is a matter of preference; however, either way, you can be more inclined to design convenience or technical flexibility.
Rendering Performance Across Email Clients
The clients have their oddities with them. Gmail also tends to strip embedded CSS and even ignores background images, whereas the rendering engine used by Outlook still uses the rendering engine of Microsoft Word, which is a relic that can frequently cause layout to become invalid in modern times. Apple Mail, however, is the most loyal to the standards of HTML, making responsive designs almost perfect. Stripo was most accurate on all three clients in our comparative tests, with a visual consistency of above 90%. MJML came second, trailing by approximately 88% with slight differences in spacing in Outlook. BeeFree and Unlayer were close behind, but they had to make some changes to the original design with custom CSS. These findings justify the fact that marketers should continue to test prior to sending. No matter how good your HTML email builder is, the only guarantee of perfection is previewing how your design behaves in every inbox before hitting “Send.”
Design and Code Factors Behind the Results
The success of any email campaign is in the background of a combination of clean design and bulletproof code. During email builder testing, we observed three common aspects that had a significant impact on the quality of the rendering and visual fidelity across Gmail, Outlook, and Apple Mail.
- HTML Structure and Inline Styling — The best builders were those who created lightweight and table-based layouts with inline CSS. These tools, such as MJML and Stripo, did well here, and their markup read identically, even in situations where clients removed external styles.
- Media Query Handling — One of the major differentiators is the responsive design. Apple Mail offered media queries perfectly well, whereas Gmail and Outlook would disregard them or alter them. This gap was overcome in builders having appropriate fallback rules, such as MJML.
- Fallback and Conditional Coding — The dependence of Outlook on the Word rendering engine requires background images and buttons to have conditional VML elements. Those cases of the builder that include built-in Outlook-specific rules, like Stripo, offered more reliable display outputs.
Strong code foundations directly translate into visual consistency, no surprises after hitting “Send.”
Usability and Workflow Evaluation
Teams not only desire good rendering but also desire time-saving tools that will cut the design friction. Stripo and BeeFree had the most intuitive interfaces with real-time previews, drag-and-drop functionality, and the possibility to export them to other popular ESPs, such as Mailchimp and HubSpot. Although it was developer-friendly, MJML required a higher technical consciousness. It is perfect when one has a team dealing with several brands and requires reusable modular components. Unlayer was, however, unique in being able to have multiple users concurrently editing, commenting, and version-controlling designs without the work of one overwriting that of the other.
The appropriate email builder must fit your workflow: visual editors are the preferred choice by non-technical marketers, and developers prefer the flexibility of open source.
Conclusions and Recommendations
The 2025 tests showed one thing: there is no builder who would be perfect, but some of them are much more reliable than others. Stripo is the most balanced code solution, with clean code, solid rendering accuracy, and simple integration. MJML is second among developers who use full control and scalability. BeeFree and Unlayer are also good choices when one is working with a smaller team and values design speed more than extensive customization. In case your branding relies on visual consistency and high deliverability, you should test all emails in a variety of clients before their launch. The discrepancies in rendering can be unobtrusive, yet they have a direct influence on engagement and trust.
