Fix Outlook Background Image Collapse in Mozilla Thunderbird — Salesforce Marketing Cloud (SFMC) Guide for Client Rendering Engine Anomalies
Email client rendering variations are caused by differences in parser engines. Legacy Outlook engines ignore modern block models, forcing layouts to stack vertically. Below is the targeted integration guide to fix Outlook Background Image Collapse inside Mozilla Thunderbird templates when using Salesforce Marketing Cloud (SFMC).
Technical Breakdown
Mozilla Thunderbird uses the Gecko which lacks support for standard rendering rules. This causes Outlook Background Image Collapse elements to break or collapse when rendering templates.
Salesforce Marketing Cloud (SFMC) Behaviors
SFMC's Content Builder wraps block exports in div frames that can break conditional comments. Additionally, AMPScript calls for personalizations can trigger parsing errors if they are inside MSO conditional comments. Ensure your AMPScript tags are evaluated *outside* the VML blocks. Test layout code regressions side-by-side using the Campaign Diff Comparator before pushing visual style changes to your production templates.
Remediation Code Block
To resolve this in Outlook desktop, you must wrap your element or cell inside a Microsoft VML (Vector Markup Language) shapes structure that declares the background image explicitly using the 'v:background' or 'v:fill' vector properties. In your Salesforce Marketing Cloud (SFMC) campaign manager, make sure you configure your links to run via %%unsubscribe_url%%.
<td background="https://example.com/bg.jpg" bgcolor="#222222" valign="top" style="background-image: url('https://example.com/bg.jpg'); background-repeat: no-repeat; background-size: cover;">
<!--[if mso]>
<v:rect xmlns:v="urn:schemas-microsoft-com:vml" fill="true" stroke="false" style="width:600px;height:300px;">
<v:fill type="tile" src="https://example.com/bg.jpg" color="#222222" />
<v:textbox inset="0,0,0,0">
<![endif]-->
<div>Overlay Content</div>
<!--[if mso]>
</v:textbox>
</v:rect>
<![endif]-->
</td>Live Code Sandbox Test
How to Deploy inside Salesforce Marketing Cloud (SFMC)
Copy the fixed VML or table wrapper code block from the remediation box above.
Create an 'HTML content' layout block in Content Builder. Paste the table structure. Enclose any custom merge tags inside standard AMPscript wrapper lines.