Fix Outlook Button Border Collapsing in Mozilla Thunderbird — Campaign Monitor 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 Button Border Collapsing inside Mozilla Thunderbird templates when using Campaign Monitor.
Technical Breakdown
Mozilla Thunderbird uses the Gecko which lacks support for standard rendering rules. This causes Outlook Button Border Collapsing elements to break or collapse when rendering templates.
Campaign Monitor Behaviors
Campaign Monitor wraps custom blocks in dynamic edit tags, which can break the column flow inside Outlook. Ensuring correct class names is essential. 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 guarantee perfectly rounded buttons inside Outlook, we must generate a hybrid VML (Vector Markup Language) roundrect structure overlaid with normal HTML tables. VML is read only by Outlook, while WebKit clients read the HTML layout. In your Campaign Monitor campaign manager, make sure you configure your links to run via [unsubscribe].
<!--[if mso]>
<v:roundrect xmlns:v="urn:schemas-microsoft-com:vml" xmlns:w="urn:schemas-microsoft-com:office:word" href="[unsubscribe]" style="height:44px;v-text-anchor:middle;width:200px;" arcsize="15%" stroke="f" fillcolor="#720e9e">
<w:anchorlock/>
<center style="color:#ffffff;font-family:sans-serif;font-size:13px;font-weight:bold;">Click Here</center>
</v:roundrect>
<![endif]-->
<!--[if !mso]><!-->
<table role="presentation" border="0" cellpadding="0" cellspacing="0" style="border-collapse:collapse;">
<tr><td align="center" bgcolor="#720e9e" style="border-radius:6px;"><a href="[unsubscribe]" style="padding:12px 24px;color:#ffffff;display:inline-block;text-decoration:none;font-weight:bold;">Click Here</a></td></tr>
</table>
<!--<![endif]-->Live Code Sandbox Test
How to Deploy inside Campaign Monitor
Copy the fixed VML or table wrapper code block from the remediation box above.
In Campaign Monitor, click the 'Source' button on your selected editable text region, or edit the template code files directly. Ensure you append the 'class="cs-editable"' flag if you need inline editing later.